Neurons are essential components of neural circuits and provide brain function organization. We previously reported that X irradiation induces apoptosis in immature neurons. To the best of our knowledge, there have been few reports investigating the effects of X irradiation on mature neurons. We analyzed the effects of X irradiation on the morphology, density and cytoskeletal proteins in dendritic spines on mature neurons. We prepared developing hippocampal neurons from 18 days embryo by using Banker's method. Neurons at 21 days in vitro were X irradiated at several doses and were immediately fixed. To evaluate the dendritic spine morphology and density, the neurons were transfected with a reporter plasmid for enhanced green fluorescent protein (GFP). Changes in the dendritic spines as a result of X irradiation were evaluated using electron microscopy. To analyze the cytoskeletal proteins within the dendritic spines, we performed immunocytochemistry to detect filamentous actin (F-actin), drebrin and PSD-95. X irradiation immediately changed the dendritic spine morphology, and the irradiated spines were significantly thinner and longer than the nonirradiated spines. X irradiation decreased the dendritic spine density in a dose-dependent manner. Electron microscopy confirmed these changes of dendritic spines by X irradiation. Immunohistochemical studies showed that X irradiation decreased the accumulation of drebrin and F-actin, but not PSD-95, within the dendritic spines. These results suggest that X irradiation immediately decreases the dendritic spine density and changes the morphology of mature neurons by reducing the abundance of cytoskeletal proteins. The abnormal dendritic spines may be associated with acute adverse effects after X irradiation in a clinical setting, although further investigations are warranted to validate these findings.