The purpose was to study on the structure of a homogeneous polysaccharide isolated from the fruiting bodies Pleurotus eryngii( PEPSC1). The PEPSC1 was isolated by hot water extraction and purified by DEAE Sepharose fast flow and Sephacryl S-500 column chromatography. The PEPSC1 was identified as a homogeneous composition by HPLC,and its molecular weight was 1. 77 × 106Da; Compositional analysis,combined with methylation analysis demonstrated that the PEPSC1 was composed of primarily mannose and glucose,their mole ratio was 1∶ 10. 3. PEPSC1 was structured by 1,6 linked glucose as backbone chain with glucose as the side chain. It contained a small amount of 1,6 linked mannose.