This study was conducted to investigate the effects of Kojongsi persimmon peels as a mushroom substrate on antioxidant activity of Pleurotus eryngii. The Pleurotus eryngii was growth on the medium with 0, 5, 10, 15, 20, 30, 40, 50% Kojongsi persimmon peels. The Pleurotus eryngii was extracted into ethanol and their antioxidant activities were analyzed. The total polyphenol content was highest in the 20% Kojongsi persimmon peels treatment (40±0.38 mg of gallic acid equivalents/100 g) than 0% Kojongsi persimmon peels treatment (21 ±0.18 mg of gallic acid equivalents/100 g). The 20% Kojongsi persimmon peels treatment showed better DPPH scavenging activity (36.81±0.23% at 1 mg/ml), ABTS scavenging activity (39.03±0.10% at 1 mg/ml) and reducing power (0.3±0.10% at 1 mg/ml) than 0% Kojongsi persimmon peels treatment and another extract ones. Overall, total polyphenol content and antioxidant activities of extracts from Pleurotus eryngii was increased dose dependently and 20% Kojongsi persimmon peels treatment was showed better total polyphenol content and antioxidant activities than 0% Kojongsi persimmon peels treatment and another extract ones.