Abstract: To prepare low molecular weight oligosaccharides from Porphyridium cruentum, the method of oxidative degradation with hydrogen peroxide under ultrasonic wave was used. The antioxidant properties of different molecular weight polysaccharides were evaluated by determining the scavenging ability of three free radicals. The results showed that the degradation ratio of polysaccharide increased and molecular weight decreased with an increase of hydrogen peroxide volume fraction(φH2O2), ultrasonic time and reactive temperature. The oligosaccharides with molecular weight 203.6, 606.6 and 730.2 ku were obtianed when φH2O2， ultrasonic time and reactive temperature were 8%, 6 h and 70 ℃, respectively. The antioxidant results indicated that the low molecular weight fragments had marked scavenging effect and the scavenging abilities of free radicals had significant differences between different molecular weight polysaccharides. Among those, the 203.6 kufragment had the best antioxidant activity.