SMMC–7721 and A375 human tumor cell lines were irradiated by ,60,Co γ-rays, X-rays and ,12,C,6+, ions with different doses up to 6Gy. Dose-survival curves were obtained ,via, colony- formation assays. RBEs of the heavy ions were calculated at different doses. For the two kinds of tumor cells, the RBE of ,12,C,6+, in fractional irradiation was higher than that in single irradiation. The RBE for SMMC–7721 cells were higher than that for A375 cells in both single and fractional irradiation, and the difference became more obvious in fractional irradiation. It is concluded that with special physical properties, the heavy ions are able to efficiently kill tumor cells with high radioresistance, and they can enhance the therapy effect and decrease suffering of the patient.