This study was conducted to investigate the effect of milk thistle plant (Silybum marianum) as a phytoantioxidant for improving the performance of Gimmizah cockerels during summer season (18 June –26 August, 2017). One hundred and twenty five Gimmizah cockerels aged 21-day were individually weighed and randomly divided into five treatment groups. Each treatment was represented by five replicates, of 5 cockerels each and housed in 25 rearing cages (open system) until the end of the experiment (91 day of age). The first group was used as a control and fed the unsupplied basal diet (without any agent). The second group was fed the basal diet supplemented with vitamin E (150 mg VE /kg diet) as an immunomodulator nutrient. The third group was fed the basal diet supplemented with 0.5g commercial silymarin (SLM) /kg diet. The fourth and the fifth groups were fed the basal diet supplemented with 12.5 and 25g of fine grind aerial parts of milk thistle plant (MTh) /kg diet as a natural source of SLM (represented 0.5 and 1g SLM /kg diet, respectively). Diets were kept isocaloric and cover nutrient requirements of the experimental birds. All birds received feed and water ad-libitum throughout the experimental period. The chemical analysis indicated that MTh contain 188.5 mg of total polyphenols and 320 mg of antioxidant activity /100g sample. Body weight of cockerels fed diet supplied with different agents was significantly increased compared to the control group through all experimental periods. In addition, the group supplied with 25 g MTh /kg diet induced the highest BW and BWG followed by those supplied with VE. Supplementation of cockerels diet with different agents significantly improved feed intake and feed conversion ratio during the whole experimental period compared to the control group. The results indicated that the relative weights of dressing, liver, spleen, bursa of Fabricius and testes significantly increased for the groups supplied with different agents compared to the control group. Moreover, the group supplied with 25g MTh /kg diet significantly increased relative weight of liver and bursa of Fabricius compared with the other experimental groups. Supplying cockerels diets with different agents significantly improved all blood biochemical constituents compared to the control group. However, diet supplied with both levels of MTh significantly improved the liver function (AST, ALT and ALP) and significantly decreased lipid profile (total lipids, triglycerides and cholesterol) compared with the group fed diet supplied with VE and the control groups. Diets supplied with different agents significantly improved TAC, GSH, MDA, lymphocyte, the heterophil to lymphocyte ratio (H/L ratio), phagocytic activity and phagocytic index compared to the control.