We have been studying LET and ion species dependence of RBE values in cell killing and mutation induction. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon (290MeV/u and 135MeV/u), neon (230MeV/u and 400MeV/u), silicon (490MeV/u) and iron (500MeV/u) ion beams, generated by Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS). Cell killing effect was detected as reproductive cell death using a colony formation assay. Mutation induction in hprt locus was detected to measure 6-thioguanine resistant colonies. The RBE-LET curves of cell killing and mutation induction were different each ion beam. So, we plotted RBE for cell killing and mutation induction as function of Z*2/B2 instead of LET. RBE-Z*2/B2 curves of cell killing indicated that the discrepancy of RBE-LET curves was reconciled each ion species. But RBE-Z*2/B2 curves of mutation induction didn\u27t corresponded between carbon- and silicon-ion beams. These results suggested that different biological endpoints may be suitable for different physical parameter, which represent the track structure of energy deposition of ion beams
