Objective：To separate and purify polysaccharide MP- I from Mytilus coruscus and to investigate its in vitro anti-tumor activity. Methods： The crude polysaccharide was extracted from Mytilus coruscus with hot water and the protein was removed using Sevage method. The polysaccharide MP- I was purified using DEAE-Sepharose and Sepharose CL-6B column chromatography. Monosaccharides analysis was carried out using thin-layer chromatography and gas chromatography. The in vitro anti-tumor activity of MP- I was assessed using MTT method. Results： The yield of MP- I was 2.14%. MP- I is mainly constituted by glucose. MP- I （at 0.5, 0. 1, 0.02 mg/ml） had different degrees of inhibitory effects on HO-8910, MCF-7, K562, and SMMC-7721 tumor cells in vitro（P〈0. 01）. The inhibitory rates of MP-I at 0. 5 mg/ml against HO-8910 and MCF-7 cells were 30.55 % and 36.38 %, respectively. Conclusion： Polysaccharide MP- I is mainly constituted by glucose and it has inhibitory effect on tumor cell lines HO-8910, MCF-7, K562, and SMMC-7721 in vitro