Abstract: Using levan from Bacillus amyloliquefaciens as raw material,acetylated levan( YL),sulfonylated levan( HL),and sulfated levan( SL) were prepared through chemical modification. Their antioxidant and antitumor activities were further studied.The result showed that after modification,the antioxidant and antitumor ability of levan from Bacillus amyloliquefaciens was improved significantly( p < 0.05),and different chemical modification had different effects.The antioxidant activity detected by ABTS and DPPH radical scavenging activity as well as iron reducing ability,was in the order of SL > HL > YL > L.The modified product SL showed the best antioxidant activity in the four polysaccharides.The antitumor activity tested on Hela in vitro was in the order of YL > HL > SL > L. The modified product YL showed the best antitumor activity in the four polysaccharides. These results demonstrated that chemical modification played an active role in improving functional activities of microbial polysaccharides.