Lemon balm (Melissa officinalis L.) belongs to Lamiaceae family. The most important used parts of this valuable medicinal species are its petioles. Its flowering branches are also being used. Its ingredients are used for treating neurological disorders, gastrointestinal and heart diseases with neurological origin. In order to examine the essential oil yield and its quality of lemon balm affected by different growth stages, an experiment was performed based on a completely randomized design with five replications at the Special Crop Laboratory, college of Agriculture, Ferdowsi University of Mashhad during year of 2012. Leaves and branches of lemon balm were collected at three stages including before flowering, full flowering and after that and then dried in shade. Results showed that growth stages had a significant effect on essential oil content of lemon balm (p≤0.01). The highest essential oil content was observed in before full flowering stage with 3.4%. The main content of essential oil was also influenced by harvest stage; at the stage of flowering, the major essential oil compounds for lemon balm were α-Tojon, β-Tojon and Camphor with 22.9, 15.5, and 14.1%, respectively. Thus, due to effect of growth stages on essential oil content and their compounds in medicinal plants such as lemon balm and in order to achieve the maximum essential oil yield, harvesting time should be determine carefully based on their growth stage.

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