Abstract: Aim To study the structure and antitumor activity of polysaccharide from Chinese yam. Methods The tube root of Chinese yam was extracted with water, from the extract, the polysaccharide of RDPS-I was separated and purified using Sevag deprotein, DEAE-cellulose and Sephadex G-100 column chromatography. Its structure was researched by PC, GC, IR, partial hydrolysis, periodate oxidation, Smith degradation, methylation analysis,₁HNMR and ¹³CNMR analysis etc. The antitumor activity in vivo also studied with animal experiments. Results RDPS-I was found to be composed of Glc, Man and Gal, in a molar ratio of 1∶0.4∶0.1, contained a backbone composed of α-D-(1,3)-Glcp and, a short branch of α-D-(1→2)-Manp-β-D-1)-Galp is attached to the main chain, the polysaccharide had a molecular weight of 41 000, ［α］_20D=+188.4° (c 0.80, H₂O) and ［η］=16.48×10^-3 (mL·g^-1). RDPS-I could significantly inhibit the cancer cell line of melanoma B16 and Lewis lung cancer in mice in vivo. Conclusion RDPS-I is a heteropolysaccharide of Glc, Man and Gal has significant antitumor activity.