Water-soluble crude polysaccharide(PIP) was extracted from cultured mycelium of the fungus Phellinus igniarius. After ethanol precipitation and sepharose CL-6B gel filtration, the fraction of PIP_1 was obtained, which was shown to be a homogeneous polysaccharide by means of high-performance liquid chromatography. The structure of PIP_1 was determined by using several methods. GC analysis indicates that PIP_1 is composed of the monosaccharides of glucose, galactose, and mannose. Their molar ratio is 3.70∶4.06∶1.00. The molar weight was estimated to be 17 kd via HPLC. IR, GC, partial hydrolysis with acid, periodate oxidation, Smith degradation, methylation, and GC-MS analysis were used for the structural analyses of PIP_1. The results show that PIP_1 has a small quantity of branch structure. The main glycosidic linkage of PIP_1 has a β-configuration. The main chain is made up of a large mass of glucose( 1→3 ) and few mannose(1→4); the side chain is composed of glucose(1→3) and galactose(1→6); the nonreduced end is composed of galactose and glucose. The side chains are branched at 6-O of glucose(1→3,6) and mannose(1→4,6). On an average, there are three branches among 20 residues. It is presumable that the existence of 1,3-linked Glc in the main and side chains is the main reason for its higher antitumor activity. After precipitation of the fungus Phellinus igniarius, the fraction of PIP_1 was obtained, which was shown to be a homogeneous polysaccharide by means of high- Performance liquid chromatography. The structure of PIP_1 was determined by using several methods. GC analysis indicates that PIP_1 is composed of the monosaccharides of glucose, galactose, and mannose. Their molar ratio is 3.70: 4.06: 1.00. The molar weight was estimated to be 17 kd via HPLC. IR, GC, partial hydrolysis with acid, periodate oxidation, Smith degradation, methylation, and GC-MS analysis were used for the structural analyzes of PIP_1. The results show that PIP_1 has a small quantity of branch structure. The The main chain is made up of a large mass of glucose (1 → 3) and few mannose (1 → 4); the side chain is composed of glucose (1 → 3) and The nonreduced end is composed of galactose and glucose. The side chains are branched at 6-O of glucose (1 → 3,6) and mannose (1 → 4,6). On an average, there It is presumable that the existence of 1,3-linked Glc in the main and side chains is the main reason for its higher antitumor activity.