Through the use of STKM-1 human stomach cancer cells, we investigated the enhancement of the anti-tumor effect and the apoptosis induction of the CDDP and caffeine combination. Even when the concentration of CDDP was low, CDDP significantly decreased the proliferation of STKM-1 human stomach cancer cells, thus confirming the synergistic effect of the CDDP and caffeine group. The apoptotic labeling index of the CDDP plus caffeine combination was significantly higher than that of the CDDP group. In conclusion, caffeine enhanced the effect of CDDP by not only inhibiting DNA repairs but also inducing apoptosis.