The subcritical water (SCW) extraction of Chaga mushroom (CM) was carried out at various temperatures (50, 100, 150, 200, 250, and 300 degrees C) and times (10, 30, and 60 min), and then antioxidant activities of the SCW extracts were evaluated by determining 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging activity, reducing power (RP), superoxide dismutase (SOD)-like activity, and total phenol content (TPC). The DPPH and ABTS radical scavenging activities, and the SOD-like activity of the extracts increased with elevated temperatures and times. For example, DPPH and ABTS radical scavenging activities, and SOD-like activity of the extracts at 250 degrees C for 60 min were 72.5, 97.8, and 92.3%, respectively, while those at 50 degrees C for 60 min were 63.2, 14.4, and 22.6%, respectively. However, the activities decreased at 300 degrees C. The highest TPC and RP were found at 250 degrees C for 30 min with values of 10.724 mg/mL and 1.063 optical density, respectively. These results indicate that SCW extraction was significantly effective on the increase of antioxidant activity of the CM.