Abstract: Four different low -molecular -weight soluble soybean polysaccharides were prepared by hydrogen peroxide with ultrasonic treatment. They were obtained with molecular weights of 93.9,43.4,18.8 and 9.6ku. Two soluble soybean polysaccharides (SSPS) were obtained with molecular weights of 155 and 2.6ku when material SSPS solution was ultrafiltrated. Subsequently every different molecular weight SSPS reacted with Fe 3+ to form its ferrous complex (SSPS-Fe ( Ⅲ ) ). Antioxidant activities of six SSPS-Fe ( Ⅲ ) complexes were evaluated in vitro by total reducing power,hydroxyl free radical scavenging capacity,sodium nitrite free radical scavenging capacity , and lipid peroxidation inhibition capacity . Results showed that six SSPS-Fe ( Ⅲ ) complexes had different antioxidant activities. Compared with five SSPS-Fe (Ⅲ) complexes,the one with the highest relative molecular mass had weaker antioxidant activity. The SSPS-Fe (Ⅲ ) complex with molecular weight of 9.6ku performed the strongest reducing power,scavenging activities to hydroxyl and sodium nitrite free radical,and lipid peroxidation inhibition. Consequently the antioxidant potential of SSPS-Fe(Ⅲ) complexes was presumably related to their relative molecular mass.