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 PIP1 was obtained, which was shown to be a homogeneous polysaccharide by means of high-performance liquid chromatography. The structure of PIP1 was determined by using several methods. GC analysis indicates that PIP1 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 PIP1. The results show that PIP1 has a small quantity of branch structure. The main glycosidic linkage of PIP1 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 among20 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.