Background and purpose: In recent years the prevalence of diabetes mellitus has been increasing worldwide. Many researchers investigated the probable effect of vitamin D on pathophysiology of diabetes mellitus but there are contradictory results. This study was designed to survey the effect of vitamin D improvement on blood glycemic control in patients with type II diabetes. Materials and methods: In a controlled clinical trial, 102 patients with type II diabetes were participated whose vitamin D levels were under 15 ng/ml. They were classified into two groups moderate deficiency (10-15 ng/ml, n=48) and severe deficiency (under 10 ng/ml, n=54). Two intramuscular (IM) injections of vitamin D (300,000 IU) were administered at a one-week interval. We measured vitamin D levels, FBS, HbA1c, and used Homeostasis model assessment of Insulin Resistance (Homa-IR) before the treatment and after 12 weeks. Paired t-test was used to analyze the pre and post intervention results and independent t-test was applied to compare the data between the two groups. Results: In both groups there were differences in vitamin D levels before and after the treatment (P=0.0001). But no significant difference was found before and after the intervention in HbA1c, FBS and Homa-IR. According to before intervention Homa-IR, significant dicreasion in HbA1c was observed only in insulin sensitive group (P=0.01). Also Patients with atleast 0.5% decreasion in HbA1c after the treatment had higher first HbA1c than no response group (P= 0.014). Conclusion: Treatment of vitamin D deficiency with intermuscular vitamin D dose not significant effect on blood glocuse though patients with higher HbA1c and also insulin sensitive show better respons to treatment.