Objective. In diabetes mellitus, vitamin [D.sub.3] deficiency affects sex hormone levels and male fertility; however, the mechanism leading to the disorder is unclear. This research was designed to investigate the mechanism of vitamin [D.sub.3] deficiency and hypogonadism in diabetic rats. Our aim was to assess serum vitamin [D.sub.3] levels and the relationship among vitamin [D.sub.3], insulin-like growth factor-1 (IGF-1), and testicular function. Materials and Methods. Rats with streptozotocin-induced diabetes were randomly divided into four groups and treated with different doses of vitamin [D.sub.3]: no vitamin [D.sub.3], low (0.025 [micro]g/kg/day), high (0.1 [micro]g/kg/day), and high (0.1 [micro]g/kg/day) with JB-1 (the insulin-like growth factor-1 receptor inhibitor group, 100 [micro]g/kg/day). The groups were compared with wild-type rats, which function as the control group. Various parameters such as vitamin [D.sub.3] and IGF-1 were compared between the experimental and wild-type groups, and their correlations were determined. Results. Twelve weeks of vitamin [D.sub.3] supplementation improved the testosterone levels, as shown by the increase in the level of serum IGF-1 in diabetic rats. Phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT), which was a downstream of the signaling pathway of IGF-1, was significantly increased after vitamin [D.sub.3] treatment. Conclusions. The study shows that vitamin [D.sub.3] may promote the expression of testosterone and improve testicular function in diabetic rats by activating PI3K/AKT via IGF-1.