Purpose : To observe the histopathological changes and caspase-3 expression in the submandibular gland in streptozotocin-induced diabetic rats after irradiation. Materials and Methods : The male Sprague-Dawley rats weighing approximately 250 gm were divided into four groups: control, diabetes, irradiation, and diabetes-irradiation groups. Diabetes mellitus was induced in the rats by injecting streptozotocin. Rats in the control and irradiation groups were injected with citrate buffer only. After 5days, rats in irradiation and diabetes-irradiation groups were irradiated with a single absorbed dose of 10 Gy to the head and neck region. All the rats were sacrificed at 3, 7, 14, 21, and 28 days after irradiation. The specimen including the submandibular gland were sectioned and observed using histopathological and immunohistochemical methods. Results : In the irradiation group, the condensed nucleus, karyolysis, and degeneration of the acinar cells and atrophy of the duct cells were observed in the early experimental phase. However, the acinar cells were found to be normal at 28 days after irradiation. In the diabetes group, the condensed nucleus, karyolysis, atrophy, and degeneration of the acinar cells were observed in the early experimental phase. However, the acinar cells were found to be normal at 21 days after diabetic state induction. In the diabetes-irradiation group, the ductal epithelial cells were predominant in their glandular tissues at 28 days after irradiation. In all of the experimental groups, the most prominent change of the acinar cells and ductal cells were observed at 14 days after diabetic state induction and irradiation. Conclusion The expression of caspase-3 in the acinar cells and ductal cells of the submandibular gland was weak after irradiation, but that in the acinar cells, ductal cells, and fibrous cells of the submandibular gland was prominent after diabetic state induction.