The aim of this study was to assess the effects of horse mint hydroalcoholic extract on blood biochemical factors and growth performance in common carp. In this research, 144 juveniles carp (mean weight 87.85±10.75 gr) were fed with commercial food enriched with horse mint hydro alcoholic extract at 0.0 (control), 0.1, 0.5 and 1 percent (per kg feed) for 45 days. After 15, 30 and 45 days of the experiment, 9 fish were caught and blood samples were taken from their caudal vein by a heparinized syringe. A significant increase was observed in glucose, ALT and ALP activity in fish treated with 0.5 and 1% of horse mint extract on day 45 (P<0.05). AST and LDH activity, cholesterol and albumin levels significantly increased in fish fed with 1% horse mint extract on day 30 and 45. Also, a significant increase was observed in triglyceride and creatinine in fish treated by extract 1% on day 30, but globulin level significantly decreased in fish treated by extract 1% on day 45 (P>0.05). Based on the results CPK only increased in fish treated by 1% of mint extract on day 15. Administration of 0.5 and 1 percent of mint extract reduced the specific growth rate, gained weight and final weight on day 30 and 45. Feed conversion ratio significantly increased on day 30 and 45. However, administration of mint extract did not have any effect on condition factor. The results of the pre-clinical study to evaluate the influence of mint extract (at concentrations of more than 5 gr) on common carp indicated that cell toxicity could be due to the administration of M. longifolia on blood biochemical indices and growth performance.