In this study the effects of cell reproductive death and DNA damage of Lewis lung carcinoma cells exposed to 89.63 MeV/u carbon ion beam were compared with that exposed to 6 MeV X-rays irradiations in order to explore the feasibility and advantage of tumor therapy by use of heavy ions beam. Results showed that the relative biological effectiveness (RBE) value of the carbon ion beams at 10% survival level was estimated up to 1.77. There are linear effects in the dose response curves both of "Tail DNA (%)" (TD) and "Olive tail moment" (OTM) obtained with comet assay for the X-rays irradiation. However, the carbon ion beams revealed biphasic dose responses including linear and exponential terms. The dose responses for the carbon ion beams had a saturated effect beyond about 8Gy. There was an inverse correlation that the high linear energy transfer (LET) carbon ion beams produced a low survival fraction at 2Gy (SF2) and a high initial Olive tail moment at 2 Gy (OTM2) compared with X-rays irradiation. In conclusion, carbon ion beams with high LET values might produce more severe cell reproductive death and DNA damage in Lewis lung carcinoma cell in comparison with X-rays, which could make the tumor therapy more effective.