This study was conducted to investigate the physiological characteristics of water and ethanol extracts from leaves and stems of Achyranthes japonica. The highest contents of total polyphenol and flavonoid compounds were 58.27 and 42.22 mg/g in water extract from leaves, respectively. The protein content was the highest at 16.42 mg/g in water extract from leaves. Ethanol extract from stems showed the highest content of reducing sugars at 11.35 mg/g. In the measurement of electron donating ability (EDA), ethanol extract from stems showed the highest EDA at 93.41% at a concentration of 1.0 mg/mL. Superoxide dismutase-like activity of ethanol extract from leaves was the highest at 8.13% at a concentration of 1.0 mg/mL. In the analysis of nitrate scavenging activity, water extract from leaves showed the highest activity at 94.90% at pH 1.2, and the activity increased as concentration increased and pH decreased. In the measurement of xanthine oxidase inhibition, ethanol extract from stems showed the highest inhibitory activity at 66.67% at a concentration of 1.0 mg/mL. Especially, nitrate scavenging activities of water extract from leaves were the highest under all pH conditions. These results verify that extracts from leaves of A. japonica have strong antioxidant activity and can be used as an effective antioxidant source for nutraceutical foods, medicines, and cosmetic stuffs.