Abstract: OBJECTIVE To study the effects of polysaccharides of Cordyceps sinensis Mycelium (cordyceps polysaccharide) on the function of T lymphocytes in tumor-bearing mice. METHODS Tumor-bearing mice were established by inoculated with S180 cells. The tumor-bearing mice were randomly divided into the tumor bearing group (physiological saline), lentinan group(1 mg·kg^-1), cordyceps polysaccharide groups in high, medium and low dose(200, 100, 50 mg·kg^-1). After celiac injection of cordyceps polysaccharide for 13 d (once a day), the number of T lymphocytes and its subgroups in peripheral blood, Th1/Th2 cytokines and transformation rates of spleen T lymphocytes were detected; and the thymus index, the spleen index and the inhibition rate of tumor were measured. RESULTS High and medium dose of cordyceps polysaccharide groups had significantly reduced growth of S180 sarcoma, increased spleen weight and spleen coefficient of tumor-bearing mice, increased the number of CD3⁺ cells (total T lymphocytes), CD4⁺CD8^- subgroup and CD4⁺CD8^-/CD4^-CD8⁺ ratio in peripheral blood, increased the level of TNF-α and IL-2 in Th1 lymphocytes, and significantly increased proliferation and transformation of spleen T lymphocytes. CONCLUSION Cordyceps polysaccharide has antitumor activity in vivo, and the underlying mechanism may relate to the regulation of the number and the immune function of T lymphocytes and its subgroup by cordyceps polysaccharide.