To study the effect of different levels of peppermint (Mentha piperita) plant powder, on immune system of broilers under heat stress condition, 192 one-day old chickens (Ross, 308) were randomly allocated to 4 dietary treatments with 4 replicates of 12 chicks each, using a completely randomized design. The four groups were characterized by a basal diet (control), basal diet supplemented with 1 and 2 percent peppermint powder and basal diet supplemented with 300 mg/kg vitamin E. Heat stress was created by setting room temperature at 34 ˚C for 8 hour/day from the 35th to the 42nd day of experiment. Results showed differences (P<0.05) for feed conversion ratio (FCR) at 21 days and body weight (BW) at 42 days of the experiment. Birds treated by 2 percent peppermint powder and 1 percent peppermint powder showed higher and lower body weight gain, respectively, at 21 days of age, when compared with birds fed basal diet and vitamin E. A significantly higher level of total Ig, IgM and IgG was found for peppermint powder than other treatment groups at 35 days and 42 days of age. Significant interactions were observed between diet and sex on IgG at 35 days of the experiment (P<0.05). There were significant (P<0.05) differences among the treatments for total white blood cells, lymphocytes, heterophils, heterophils to lymphocytes ratio at 42 days of experiment and 2 percent peppermint powder increased total white blood cells values compared to basal diet and vitamin E. The peppermint powder significantly made a difference for serum concentrations of total protein, albumin, globulin, triglyceride, total cholesterol, high-density lipoprotein-cholesterol (HDLC), low-density lipoprotein-cholesterol (LDLC) and very low-density lipoprotein-cholesterol (VLDLC)at 21days and 42 days of age (P<0.05). Blood serum concentration of HDLC increased by peppermint powder treatment, whereas they were lower for basal diet and vitamin E at 42 days of age. Liver weight was higher and lower in 1 percent peppermint powder and basal diet treated groups, respectively (P<0.05). In general, results indicated that supplementation of peppermint powder in the diet did not improve bursa of fabricius and spleen weight of broiler chicken, but ha an antioxidative potential to improve oxidative stability and immune response. Manuscript profile