Abstract: Methanol extract of old leaves of Toona sinensis (A. Juss.) Roem. was separated into four subfractions of different polarities based on bioassay-guided method. The antioxidative activities and the related responsible compounds in T. sinensis were studied by evaluating reducing power, ferric reducing ability of plasma (FRAP) value and 1,1-diphenyl-2-picrylhydrazyl (DPPH)radical scavenging capacity of four subfractions with different polarities. The results showed that reducing power, FRAP value and DPPH radical scavenging capacity of ethyl acetate subfraction at a concentration of 50 mg/L had the strongest antioxidant activities of 467.53mg/g (ascorbic acid) Vc equivalents, 10578(μmol/L)/g FeSO₄7 H₂O equivalents and92.84%, respectively. All these antioxidant values of ethyl acetate subfraction were much higher than BHT at the same concentration. The correlation study showed that the powerful antioxidative activity of T. sinensis was mainly resulted from high contents of flavonoids. The antioxidants mainly consist of four [JP2]flavonoid glucosides. These results laid a solid foundation for further researching and developing T. sinensis .

Key words: antioxidation, Toona sinensis (A. Juss.) Roem., bioassay-guided method, evaluation