In order to evaluate the effect of supplemental vitamin C (VC) levels on performance and immune response of broiler chicks, 300 day-old Ross male broiler chicks were allocated in complete randomized design with 5 treatments(500 and 1000 ppm in diet, 500 and 1000 ppm in drinking water and control). Immunity was assessed as antibody production to Infectious Bronchitis virus (IBV) and Newcastle Disease virus(NDV), mitogenic response to phytohemagglutininA (PHA) and concanavalin A(conA), cutaneous basophil hypersensitivity (CBH) to PHA. VC ( 500 and 1000 ppm in drinking water and 1000 ppm in diets) increased (P<0.05) daily weight gain (0-21). Maximum body weight gain (42d), daily weight gain(21-42d) and efficiency of food utilization were obtained in chicks drank with supplemented 1000 ppm VC. Mitogenic responses were improved (P<0.05) by supplemented 1000 ppm soluble VC for PHA in pure lymphocyte and whole blood . CBH to PHA was significantly increased (P<0.05) in chicken drank 500 and 1000 ppm VC. Lymphoid organ weights were not influenced by VC .Humoral immunologic response showed that antibody titer to NDV(HI) and IBV(Elisa) were highest (P<0.05) in groups receiving 500 and 1000 ppm VC in drinking water. Overall, the results of this experiment showed that VC in drinking water, improved some of performance parameters and immune response in broiler chicks, respectively specially in early ages. Keywords: Vitamin C, Immune response, performance, broiler chicks

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