Abstract: To investigate the effect of Methylcobalamin on the secretion of testosterone in diabetic rats. 30 male Wistar rats were randomly divided into 3 groups: normal group, diabetic rats treated with Methylcobalamin group and diabetic rats （DM） group. The diabetic rat model was induced by STZ. Methylcobalamin （500 μg/kg） was injected intramuscularly in Methylcobalamin treated group and same volume of saline was injected in the other groups. At the end of the 60 days trial, the blood glucose, body weight, testosterone and hCG binding site of testis were measured. The results showed that body weight of normal group was significantly higher than that of other groups; blood glucose of normal group was lower than that of other groups; there were no significant differences in body weight and blood glucose between methylcobalamin treated group and DM group; the testosterone and hCG binding sites of normal group were significantly higher than those of other two groups （P ＜0.01）, the testosterone and hCG binding site of methylcobalamin treated group were higher than that of DM group. The results suggest that diabetes has an obvious effect on testosterone secretion and hCG binding site, both of them being reduced; whereas, methylcobalamin may protect Leydig cells from damage of diabetes, but the mechanism is unclear. Moreover the methylcobalamin has no effect on body weight and blood glucose of diabetic rats.