Grape seed proanthocyanidin extract (GSPE) has been widely used as a human food supplement for health promotion and disease prevention. However, there was little information regarding its application in animal nutrition. The aim of the current study is to determine the effect of GSPE at different concentrations on chicken performance, and the status of antioxidant/oxidant system after the Eimeria tenella infection. In the first experiment, GSPE incorporated in the diet at 5, 10, 20, 40, and 80 mg/kg significantly decreased mortality and increased weight gain after the E. tenella infection, and the protective effect of GSPE was dose-dependent. The lowest mortality and the greatest growth gains were recorded in the group of birds fed with GSPE between 10 to 20 mg/kg. In the second experiment, 12 mg/kg of GSPE supplementation in the diet significantly reduced the mortality and lesion scores in birds after the infection with 5 x 10(4) and 1 x 10(5) oocysts of E. tenella. The weight gains also improved significantly. After the oral infection with 5 x 10(4) and 1 x 10(5) of E. tenella, analysis of the status of antioxidant/oxidant system revealed that plasma NO increased significantly from 7.11 to 21.31 micromol/L, plasma superoxide dismutase (SOD) decreased from 126.55 to 111.14 U/mL, and malondiadehyde increased, suggesting oxidative stress was increased in circulation. However, supplementation of 12 mg/kg GSPE reduced the level of plasma NO from 21.31 to 14.73 micromol/L and increased plasma SOD activities from 111.14 to 133.27 U/mL. The effects of incorporation of GSPE into the poultry diet on the concentration of plasma NO, malondiadehyde, and SOD indicated that the lower concentration of dietary GSPE was able to restore the balance of antioxidant/oxidant system that was exerted by the oxidative stress after the parasite infection. The current results suggested GSPE can act as an antioxidant in diet to improve the performance of broiler chickens and remedy the clinical symptoms caused by the oxidative stress of E. tenella infection.