Objective Low concentrations of plasma vitamin D (25(OH)D) have been associated with the development of metabolic syndrome, obesity and diabetes. The objective of this study was to investigate the impact of vitamin D concentration on the relationship of body mass index (BMI) status and insulin resistance in type 2 diabetes. Methods Retrospective study of all patients who had attended the diabetic clinics in a Eulji hospital between January 1st and July 31th on 2013 and whose concentrations of vitamin D, fasting glucose, and fasting insulin had been determined (n = 257, 44.4% women). We used the BMI for obesity and the HOMA-IR index for insulin resistance. Results The negative correlation between serum vitamin D and BMI with type 2 diabetes was significant (r = 0.268, P < 0.0001). And the negative correlation between vitamin D and insulin resistance in type 2 DM was also remained significant (r = 0.124, P < 0.0001). Multiple regression analysis demonstrated that vitamin D was not associated with BMI (P = 0.681) but associated with insulin resistance (P < 0.042). Conclusion As conducting a study of type 2 diabetes, plasma vitamin D has negative correlation with BMI. And plasma vitamin D has strong association with insulin resistance. Therefore, we conclude that hypovitaminosis D has a valuable attributable risk for type 2 diabetes. Plus, further prospective and not cross-sectional studies will be needed to confirm whether the supplement of vitamin D would be needed for improving the status of diabetes in type 2 diabetes patients.