Objective The appropriate dietary management for patients with type 2 diabetes (T2DM) is unclear. We investigated whether a vegan diet improves glycemic control and metabolic parameters including plasma total cholesterol, triglyceride, HDL-cholesterol levels, and waist circumference compared to standard diet following ADA guidelines. Methods 113 patients with T2DM to vegan diet (n = 58) or diet following American Diabetes Association (ADA) guidelines (n =55) were assigned randomly. Dietary intake was assessed by using a 24-h recall. The compliance score was estimated by daily self-assessment based on meals diary. Results 79% and 87% of participants in ADA group and vegan diet group were females, respectively (P = 0.330). The baseline characteristics were similar between two groups except for waist circumference (P = 0.044). Mean age was comparable, 58.4 in ADA group and 56.5 in vegan diet group (P = 0.256). The compliance score was lower in vegan diet group (P = 0.001). Estimated energy intakes per day were not different in both group (P = 0.228). The reduction in HbA1C was observed in both groups, although the degree of reduction was significantly greater in vegan diet group (ADA group; from 7.4 to 7.2%, p trend = 0.031 vs. vegan diet group; from 7.7 to 7.1%, P < 0.001).There was no significant change of plasma total cholesterol, triglyceride, and HDL-cholesterol levels after 12-week trial. Waist circumference was decreased only in vegan diet group (P < 0.001) but not in ADA diet group (P = 0.352). Conclusion A vegan diet was superior to ADA diet in improving glycemic control and lowers waist circumference in patients with T2DM.