Isolation, purification and antioxidant activity in vitro of polysaccharide from Ganoderma lucidum were studied. Polysac-charide was extracted from G. lucidum, and three fractions were isolated through ion exchange chromatography and allyl dextran gel chromatography from them, including GLPa-2, GLPb-1, and GLPc. The weight-average molecular weight was detected by gel permeation chromatographic, and then monosaccharide composition was detected by gas chromatography. The results were described as follows. The component parts of polysaccharides were composed of glucose, arabinose, xylose and mannose, with different containing rates, and the average molecular weight of GLPa-2, GLPb-1, GLPc was detected to be 3.65×10⁵ Da, 3.87×10⁴ Da, 1.38×10⁴ Da, respectively. Moreover, a dose-effect relation between the concentration of polysaccharides fraction and the eliminating ratio was also found, and GLPa-2, composed of glucose, arabinose, xylose, and mannose, the largest molecular among the three ones, showed the highest antioxidant activity. Thus it turned out that the antioxidant activity of polysaccharides from G. lucidum was related to their composition.