The purpose of the study was to investigate the comparative beneficial effects of dietary supplementation of silymarin (Silybum marianum) and zinc oxide (ZnO) in broilers during the finishing stage exposed to heat stress (HS). Two hundred and seventy broiler chicks were randomly assigned to 6 dietary groups: three dietary groups in thermoneutral temperature (TN) and three dietary groups under high temperature. One group of birds received a basal diet only (control group), second group of birds received a diet supplemented with silymarin at the level of 10g/kg and third group of birds received a diet supplemented with ZnO at the level of 60 mg/kg in both TN and HS birds. It was found that supplementation of ZnO significantly (P<0.05) improved feed intake, dressing percentage, body weight, feed conversion ratio, blood parameters and nutrient digestibility at both levels of thermal zones. The results of silymarin on growth, nutrient digestibility and hematological parameters were comparatively lower (P<0.05) in broilers during TN and HS zones. The current results indicate that the supplementation of feed with silymarin and ZnO improved the growth performance, blood parameters and nutrient digestibility in broiler during heat stress, however, the positive effect of ZnO was better than silymarin.