Abstract: OBJECTIVE To investigate the effect of recombinant snake venom cystatin(sv-cystatin) on apoptosis of tumor cells and its molecular mechanism. METHODS B16F10 and MHCC97H cells were incubated with recombinant sv-cystatin(10, 25, 50, 100, 200 mg·L^-1). Melanoma tumor-bearing mice model was established by inoculating in the lateral tail vein of C57BL/6 mice with B16F10 cells, and recombinant sv-cystatin was administered at 25, 50 mg·kg^-1. MHCC97H cells were inoculated subcutaneously(s.c.) into BALB/c-nude mice, and recombinant sv-cystatin was administered at 25, 50, 100 mg·kg^-1. TUNEL analysis was used to detected the apoptosis of B16F10 and MHCC97H cells. The mRNA and protein expression of Bcl-2, Bcl-w and Bax, and protein expression of Cyto C were measured, respectively. Activity of caspase-2 and caspase-3 were assessed. RESULTS Compared with control group, the apoptosis of B16F10 and MHCC97H cells was significantly upregulated by treatment with recombinant sv-cystatin at 25, 50, 100, 200 mg·L^-1(P<0.05). The apoptosis of B16F10 cells in the lung colonization of C57BL/6 mice(50 mg·kg^-1), and the apoptosis of MHCC97H cells in the primary tumor tissues of BALB/c-nude mice(100 mg·kg^-1) were higher than those in control mice(P<0.05). The mRNA and protein expression of Bcl-2 and Bcl-w in B16F10 and MHCC97H cells were decreased following treatment with recombinant sv-cystatin, compared to controls(P<0.05), while the expression of Bax and Cyto C was significantly raised(P<0.05). Activity of caspase-2 and caspase-3 was significantly increased(P<0.05). CONCLUSION Recombinant sv-cystatin can promote the apoptosis of tumor cells, which contribute to anti-tumor effect.