Objective: The trial involved 240 Ross 308 broiler chicks in order to investigate the effects of Mentha pulegium L. (pennyroyal) on performance, carcass traits and immune responses in broiler chickens. Methods: Birds were assigned to 4 treatments: control feed, antibiotic group receiving 4.5 mg/kg flavophospholipol, and 5 and 10 g/kg pennyroyal powder added to the basal diet. Body weights of broilers were determined at d 1, 14, 28 and 42, feed intake was determined at the same periods, and feed conversion ratio was calculated accordingly. At day 42, two birds per replicate were slaughtered for the determination of carcass traits. Antibody titers against newcastle, influenza viruses and sheep red blood cell (SRBC) were determined. Results: Performance, Internal organ weights and carcass characteristics were not significantly influenced by the dietary treatments at day 42. Humoral immune responses were not affected by dietary treatments.Conclusions:In conclusion, the results of this study showed that addition of pennyroyal powder seem not to have a positive influence on growth performance of broiler chicks.