The aqueous extract of Cordyceps sinensis (Cs), one of the traditional Chinese medicines, has been used for the treatment of a wide range of disorders for centuries. It is generally accepted that its cultivated Cs fungi possess the same functions as Cs natural herbs. Although polysaccharide from Cs is one of its bioactive compositions, its antitumor ability has not been confirmed. In the present study, we investigated the effects of the exopolysaccharide fraction (EPSF) of a cultivated Cs fungus on c-Myc, c-Fos, and vascular endothelial growth factor (VEGF) expression of tumor-bearing mice. The mice (C57BL/6) were administered three different doses of EPSF peritoneally every 2 days, starting from the day of implantation of B16 melanoma cells through their tail veins for 27 days (14 times). Sections from mouse paraffin-embedded liver and lung tissues were subjected to immunohistochemical analyses. The results of c-Myc, c-Fos, and VEGF expression were analyzed using SimplePCI image analysis software. The c-Myc, c-Fos, and VEGF levels in the lungs and livers of EPSF-treated mice were found to be significantly lower than those of untreated mice (p<0.05). This suggests that EPSF had inhibited tumor growth in the lungs and livers of mice, and that it might be a potential adjuvant in cancer therapy.