This study examined the antioxidant activity of ethanol extract and fraction from Castanea crenata inner shell in vitro. Antioxidant activity was evaluated through total polyphenol and flavonoid contents and DPPH radical scavenging ability using electron spin resonance (ESR). The total polyphenol contents of the 70% ethanol extract and ethyl acetate fraction were 39.76 mg GAE/g and 323.92 mg GAE/g (p < 0.05). The total polyphenol contents of the ethyl acetate fraction were higher than other fractions. Furthermore, the total flavonoid contents of ethyl acetate were higher than other fractions at 13.12 mg QE/g (p < 0.05). In DPPH radical scavenging ability measurement, the RC 50 value of ethyl acetate was lower than ascorbic acid at 2.27 μg/mL (p < 0.05). In conclusion, this result showed that the ethyl acetate fraction of Castanea crenata inner shell 70% ethanol extract has high antioxidant activity.