According to the method established previously, the osteoclasts were released and then settled on to glass slides. The numbers of apoptotic osteoclasts after in vitro γ-irradiation (0.5,1,3, 5 Gy) were measured by fluorescence staining assay. The results are that control samples and samples irradiated at 0.5 Gy could be distinguished in a statistically significant way. This suggests that osteoclasts are sensitive to irradiation and low doses can induce apoptosis. Furthermore, the percentage of apoptotic cells at 3 Gy reached the peak, this could be provided as a model for studying the molecular mechanism of apoptosis.