Introduction: Cancer has a difficult therapeutic process. Treatment procedures of cancer cause the child be often tired and languishing. These children are also more prone to infections and frequently are hospitalized that leads to the separation of children from their families and the community. Pain, fatigue, anxiety, and other symptoms are common in those experiencing cancer. Satin, Linden and Philips (2009), in a meta-analysis study found that the risk of death in cancer patients that report depressive symptoms is more than non-depressed patients, The rate of death in patients who show some symptoms of depression is 25%, and in patients with mild or major depression is 39% more than other cancer patients. Since childhood cancer is a life-threatening disease, the role of parents, especially mothers as a primary carer, are increasingly highlighted and because of changes in the role of parents, parents are facing with multiple and complex problems. Parents often have feelings of depression and anxiety. Patient and caregiver characteristics, stressors, stress appraisal, stress coping methods, and social support influence caregivers' quality of life. Studies show that mothers of children with cancer report more perceived stress than mothers of healthy children and even mothers of children with Down syndrome. Studies also show that parents of children survivors of cancer show significantly higher levels of post-traumatic stress symptoms compared with healthy children parents. Play therapy allows children to encounter with emotional needs and doubts, and provide an opportunity to find ways to cope with their hospital circumstances. On the other hand children need sufficient time to engage emotionally with their parents, and parents should know that how interact with their children in an efficient way. The way in which mothers interact with their children is very important. Since play therapy is one of the best ways to get into the child's world and it can also have a positive impact on the children's physical and mental health, so if the mothers enter this field, the play can be an effective way for mothers to interact with their children. Material and methods: Participants: Participants were recruited from the inpatient oncology unit at Dr. Sheikh Hospital in the city of Mashhad, Iran, over a 5 month period. Patients were included in the study if their diagnosis was cancer, they were between 7-12 years old, their mothers were primary caregiver and were capable of participating in a 1-hour 8 sessions of play therapy. So 14 children diagnosed with cancer and their mothers (28 individuals) entered in final program. Measures and procedure: The instruments used to measure depression symptoms, perceived stress and Parent-Child Relationship were Perceived Stress Scale (PSS), Child depression inventory (CDI), and Parent-Child Relationship Scale (PCRS). Perceived stress scale: This scale has three versions with 4, 10 and 14 items respectively. Cohen et al (1983), conducted a study to evaluate the psychometric properties of all these three versions. Cronbach's alpha of 14-item version was 0/84 and 0/86. In a study carried out by Ghorbani et al (2002) in the Iranian society, Cronbach's alpha was 0/81. Child depression inventory: It is a self-assessment scale developed by Kovacs (1977), which was designed to assess signs of cognitive, behavioral and emotional depression in children and adolescents aged 7 to 17. In Dehshiri, Najafi, sheikhi and Habibi Askarabad (2009) test-retest reliability and internal consistency of the questionnaire respectively was 0/82, 0/83 Parent-child relationship scale: This scale was developed by Pianta for the first time in 1994. It contains 33 items that assesses parental perception of their relationship with their child. It is applied for children aged 3-12. This questionnaire was translated by Tahmasian in 2007 and its validity was assessed by experts. First the participants answered the PSS, PCRS and CDI questionnaires as pre-test. Then the individuals in intervention group received 8 sessions of parent – child play therapy, at the end, both of intervention and control group filled post-test questionnaires. Result: Whit regard to depression, the analysis of covariance data showed a significant change between pre-test and post-test (F(1,11)=92/34, p < 0/001, =□0/89). In other words, there is a significant difference in depression scores between the subjects of experimental and control groups at post-test. Post-test scores of Depression in the experimental group were significantly lower than the control one. In the case of perceived stress variable, after controlling the effect of pre-test scores, the efficacy of treatment in post-test scores was not significant (F(1,11)=1/40,p < 0/05, =□0/11). So, there is no a significant difference between the subjects of experimental and control groups in mean scores for perceived stress of mothers in the post-test. For child-parent relationship, there is a significant relationship between pre-test and post-test scores of this variable. The results in the table above shows After controlling effect of pre-test scores, the efficacy of treatment for child-parent relationship in post-test scores are also significant(F(1,11)=12/84,p < 0/01, =□0/53). So, there is a significant difference between the mean scores of parent-child relationship in the post-test in subjects of the experimental and control groups. Discussion and conclusion: Childhood cancer is one of the most painful diseases for children and families. This illness can threaten the mental health of the child and his family. Children encounter with painful side effects of cancer medical treatments, such as nausea, vomiting, hair loss, fatigue, pain and etc. And because of diseases children cannot participate in peers and family communities, their life change considerably. Children with cancer are more likely to experience symptoms of depression and, compared with other children, and the risk of death in the children that represent more symptoms of depression is more than other children. Also psychological conditions of the children’s’ parents affect the child behaviors and emotions. Children who have a better relationship with their parents and their parents experience less stress, have less behavioral problems and greater adherence to treatment. Psychological interventions that aim these problems are great help for these children and their parents. One of these interventions is play therapy that is enjoyable activity for children. On the other side, involvement of parents in treatment can lead to better results. Current study also aims to reduce the symptoms of childhood depression, reduce perceived stress on their mothers and improve parent - child relationship used parent child play therapy. Parent-child play therapy involves child and his parent in enjoyable and purposeful activities, to express emotions and thoughts in a secure atmosphere. Training of relaxation techniques to both child and mother and to instruct empathy and behavior management techniques to mother helps to solve parent and child psychological problems. Intervention that used in current study is effective in reducing symptoms of depression and improve parent – child relationship, but it is not effective on reducing of mothers Perceived stress

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