Abstract: β-sitosterol was extracted from Polygonum tinctorium Aiton by ultrasonic wave method and its content in source plant was also determined in this paper. Present study also conducted the research of antioxidant and free radical scavenging activities of β-sitosterol,as well as comparing with the other normal antioxidants,the antioxidative effect of β-sitosterol in the preservation of lard and sesame oil,and the synergistic antioxidative effect of β-sitosterol mixed with citric acid or vitamin C. The results showed that the content of β-sitosterol in Polygonum tinctorium Aiton was 0. 893 %. The extracts of β- sitosterol could effectively scavenge hydroxyl radical and superoxide anion radical, better than benzoic acid and mannitol and weak than VCin hyper-concentration. 0. 08 % β- sitosterol could effectively inhibit the lard and sesame oil preoxidation, and more effective when mixed with citric acid or vitamin C.