Solanum lyratum Thunb (Solanaceae) has been widely used for cancer as a folk remedy in Chinese traditional medicine. In this study, the main active fraction n-butanol extract from S. lyratum (BESL) was evaluated for the therapeutic efficacies on mice transplantable tumor and immunomodulatory potentials on the immune response in tumor-bearing mice. The effects of BESL on the growth of mouse transplantable S180 sarcoma, splenocyte proliferation, the activity of natural killer (NK) cells and cytotoxic T lymphocytes (CTL), production of cytokines from splenocytes, and serum antigen-specific antibody levels in S180-bearing mice were measured. BESL could not only significantly inhibit the growth of S180 sarcoma transplanted in mice, but also remarkably promote splenocytes proliferation, NK cell and CTL activity, interleukin-2 and interferon-γ production from splenocytes, and serum antigen-specific antibody levels in tumor-bearing mice (P < 0.05, P < 0.01, or P <0.001). The results suggested that BESL might exhibit antitumor activity by improving immune response, and it could act as antitumor agent with immunomodulatory activity. This study provided evidence to understand the therapeutic effects of S. lyratum for treatment of cancer and a natural product to further researches to be developed as a cancer chemopreventive agent.