Diabetic patients are more at risk for different cardiovascular, neurological, renal, and eye complications than people. Since glucose is replaced competitively in many chemical reactions and prevents and this prevents nonenzymatic glycosylation of proteins- particularly hemoglobin and lipoproteins, it seems that vitamin C is effective in preventing diabetic complications. In the current study, we investigated the effect of supplementary vitamin C on diabetic patients. In a clinical trial, 31 patients [13 males and 17 females] with a mean age of 50.9 ± 4.9 years suffering from type 2 diabetes were selected using convenient sampling. Then they were divided into two groups of case and control. The case group received 800 mg of vitamin C daily for two months and the control group received this amount of placebo. The amounts of glucose, insulin and lipids in the blood of the patients were measured before and after the intervention. Then, data were analyzed using paired and independent t tests. There was no significant difference among anthropometric indexes, food intake, and consumed drugs of the subjects during the study. Vitamin C consumption decreased plasma insulin level [28.25 ± 2.37 and 27.8 ± 2.56, p <0.016] and plasma triglyceride concentration [279.41 ± 194.99 and 212.48 ± 2.18 p< 0.03]. The decrease in the levels of plasma glucose and other lipid profiles was not significant for two months of vitamin C consumption. Conclusion: It seems that more research is required to investigate the impact of vitamin C supplement on glycemic control and blood lipids of diabetic patients.