The anti AIDS effects of marine polysaccharide drug 911 both in vitro and in vivo were studied in this paper. The results indicated that 911 inhibited both acute and chronic infection of HIV 1 to MT 4 and H 9 cells with its half effect concentration (EC 50 ) at 4.44 mg·L 1 and 0.32mg·L 1 respectively in vitro. While the half toxic concentration (TC 50 ) of 911 to MT 4 cells was 7.78 kg·L 1 and that to H 9 cells 7.47g·L 1 . And the in vivo study showed that oral administration 911 reduced the virus titer and decreased plasma RNA copies in the macaques at doses of 64mg·d 1 and 320mg·d 1 . The protective effect on CD 4 + cells and antibody increasing activity of 911 were also found. Furthermore, 911 significantly interfered the adsorption of HIV to MT 4 cells and suppressed activity of reverse transcriptase. These findings indicated that 911 inhibited the viral replication both in vitro and in vivo by blocking the adsorption of virus to host cells and counteracting the activity of reverse transcriptase and modulating immune function as well.