Abstract: Objective To observe apoptosis and expression of Bcl-2 and Bax in cochlear tissue of spontaneous diabetic mice. Methods Nonobese diabetic (NOD) female mice were allocated into the 4weekdiabetic group (group D4), 12weekdiabetic (group D12) and nondiabetic control group (group C). Acridine orangepropidium iodine (AOPI) fluorescent staining and TdTdUTP terminal nickend labeling (TUNEL) were performed to detect cochlear apoptotic cells. Immunohistochemical assay and Western blot assay were used for detecting the expression of Bcl2 and Bax in diabetic cochlear tissue. Results Apoptotic cells could be observed in the Cortis organ, spiral ganglion, stria vascularis, spiral ligament, and support cells of the diabetic groups. The apoptotic rates of hair cells in group D4 were obviously lower than those in group D12 (P＜0.05). In group D4, Bax was positively expressed in cytoplasm of Cortis organ, spiral ganglion, stria vascularis and spiral ligament; and Bcl2 was positively expressed in both nuclei and cytoplasm of above sites. In group D12, Bax was positively expressed in both nuclei and cytoplasm of above sites, while Bcl2 was expressed only in cytoplasm. Compared with group C, the expression of Bax increased significantly in both group D4 and group D12 (P＜0.05) while that of Bcl-2 increased significantly in group D4 (P＜0.05) but decreased in group D12 (P＜0.05). The expression of Bax in group D4 was significantly lower than that in group D12 (P＜0.05) while that of Bcl2 was reverse (P＜0.05). Conclusion Apoptosis exists in cochlear tissue of spontaneous diabetic mice. The injury mechanism of apoptosis is related with the increased expression of Bax and decreased expression of Bcl-2.