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Physical and biological properties of heavy-ion beams are unique as compared with that of x-and gamma-rays. Depth-dose distribution in a patient body exposed to heavy-ion beams shows a Bragg peak which is a region of high LET (linear energy transfer). Preclinical radiobiological studies for heavy-ion radiotherapy have solved some biological questions. Radiosensitivity of cells increased with increasing LET of heavy-ion beams. The maximal efficiency of inactivation and mutation of mammalian cells by ion beams were obtained at LET around 100 keV/micrometer. The radiosensitivity of wild-type cells against X-rays may be higher than that of mutant p53 cells. However, p53 independent cell killing observed for carbon-ion beams. There is a possibility that heavy-ion beams are able to overcome the resistivity of cells caused by mutation of p53 gene, hypoxic conditions, cell cycle conditions and repairability. Mutation frequency of cells depends on the kind of heavy-ion beams. Therefore heavy-ions should be selected to be confirmed the low frequency of radiation injury and the high tumor control rate. Clinical trials were started using carbon-ion beams in June 1994 at the National Institute of Radiological Sciences, Chiba, Japan. The patients of 1,042 had been treated by August 2001. Optimum dose was estimated through Phase I/II trials for bone/soft tissue tumors, head and neck tumors, non-small cell lung cancer, liver cancer, etc.