Background: Diabetes mellitus continues to be a public health concern. Vitamin D had sparked widespread interest in the pathogenesis and prevention of diabetes. The aim of this study was to investigate the effect of vitamin D (deficiency & treatment) with alteration in fasting plasma glucose, insulin resistance in alloxan injected rat. Materials and methods: The experiment was carried out using 40 male albino rats (Sprague Dawley) weighing 150±10g.  Animals were randomly divided into three groups; first group fed standard diet as a negative control group. Diabetic group injected subcutaneously by alloxan, and fed on standard diet. The third group fed standard diet without vitamin D for two weeks. After that glucose and insulin were determined in each rat of all groups to insure alteration in fasting plasma glucose, insulin resistance, Homeostasis model assessment insulin resistance (HOMA-IR)was calculated. Then the third group was divided to two subgroups. The first subgroup fed basil diet with required vitamin D; while the second subgroup fed standard diet with double dose vitamin D. At the end experiment (4 weeks), glucose, insulin, lipid profile, liver and renal functions were determined in blood and serum, while   (HOMA-IR)and LDL were calculated for normal, diabetic group and both treatment subgroups. Results: Vitamin D deficiency group had the nearest results to the diabetic group injected with alloxan group in: insulin, glucose and HOMA-IR. Other groups had lower level than the other two groups in the same parameter. Our data explained the improvement in glucose level after feeding with vitamin D. Diabetic group injected with alloxan had increased in liver enzymes, renal function and lipid profile compared with other groups and showed variable changes in histopathological examination. Conclusion: Vitamin D deficiency status is associated with a higher risk of type 2 diabetes. Vitamin D improves glucose tolerance and insulin sensitivity. Vitamin D has also been shown to reduce the risk of diabetes associated complications.