BACKGROUND: The application of medicinal herbs with high anti-oxidant properties could reduce the detrimental effects of oxidative stress and free radicals under heat stress and improve the performance of broilers. OBJECTIVES: The effects of rosemary powder and extract on growth performance, carcass characteristics and blood parameters of broilers were evaluated under heat stress. METHODS: For studying the effect of experimental treatments including control, different levels of rosemary powder (5, 10 and 15 g/kg), rosemary extract (3.5,7 and 10.5 g/kg) and Vit-Asp (1 g/L) on growth performance (feed intake, body weight and feed conversion ratio), carcass characteristics and blood parameters (cholesterol, triglyceride, hematocrit, glucose and heterophil to lymphocyte ratio), 160 one-day old chicks (Ross male-308) were used applying a randomized complete block design with 8 treatments, 4 replicates and 5 chicks per replicate during a 42-day period. Heat stress schedule was 32±2ºC as cyclic from 10.00 AM to 6.00 PM. RESULTS: Rosemary had significant effects on feed intake and final live weight of chicks (p<0.05) but not on feed conversion ratio (p>0.05). Significant effects from rosemary on glucose and heterophil to lymphocyte ratio were found (p<0.05). The effects of rosemary on carcass characteristics, cholesterol, triglyceride and hematocrit were not significant (p>0.05). CONCLUSIONS: In general, inclusion of rosemary in broilers diet under heat stress improved the performance because of anti-oxidant effects.Keyword: Rosemary, performance, blood parameters, heat stress, broilers