Inonotus obliquus (persoon) Pilat (Chaga, in Russia, kabanoanatake in Japan) is a fungus having been used as a folk medicine in Russia and said to have many health beneficial functions such as immune modulating and anti-cancer activities. In the present study, the antioxidant activity of hot water extract (decoction) of Chaga was precisely compared with those of other medicinal fungi (Agaricus blazei Mycelia, Ganoderma lucidum and Phellinus linteus) showing Chaga had the strongest antioxidant activity among fungi examined in terms of both superoxide and hydroxyl radicals scavenging activities. Further determination of the antioxidant potential of isolated fruiting body (brown part) and Sclerotium (black part) revealed the 80% MeOH extract of fruiting body had the highest potential as high as that of Chaga decoction. Finally, seven antioxidant components were isolated and purified from the 80% MeOH extract of Chaga fruiting body, and their chemical structures were determined as small phenolics as follows: 4-hydroxy-3,5-dimethoxy benzoic acid 2-hydroxy-1-hydroxymethyl ethyl ester (BAEE), protocatechic acid (PCA), caffeic acid (CA), 3,4-dihybenzaladehyde (DB), 2,5-dihydroxyterephtalic acid (DTA), syringic acid (SA) and 3,4-dihydroxybenzalacetone (DBL). Notably, BAEE was assigned as the new compound firstly identified from the natural source in the present study.