We studied the localization of phosphorylated H2AX in cultured human fibroblasts after irradiation with heavy ion beams. Asynchronous human normal fibroblasts (NB1RGB) were irradiated with X-rays, carbon ion beam (LET is about 88 keV/µm), Si ion beam (220 keV/µm) and Fe ion beam (440 keV/µm) at room temperature. Phosphorylation of H2AX in irradiated cells was measured from 0 to 24 h after irradiation monitored by using flow cytometry. Phosphorylated H2AX increased just after irradiation of each radiation and reached maximum around 30 min. Phosphorylated H2AX was then decreased quickly for cells irradiated with X-rays but presented longer for Fe beams. Foci of γ-H2AX on cell nucli were counted under the laser scanning confocal microscopy. Focus was not visible just after X-irradiation, but obvious after Fe ion irradiation. Number of foci per nucleus was increased with increasing dose of each radiation at 30 min after irradiation.