Controversy exists among trials assessing whether prolonged antioxidant vitamin supplementation improves endothelial function in type 2 diabetes mellitus (T2DM) subjects. The aim of this study was to systematically review and quantify the effect of antioxidant vitamin supplementation on endothelial function in T2DM subjects. MEDLINE, Cochrane, Scopus and Web of Science were searched up to February 2013 for randomized controlled trials assessing the effect of antioxidant vitamin E and/or C supplementation on endothelial function in T2DM subjects. Ten randomized controlled trials comparing antioxidant vitamin-supplemented and control groups (overall n = 296) met the inclusion criteria. Post-intervention standardized mean difference (SMD) in endothelial function did not reach statistical significance between groups (0.35; 95% confidence interval = −0.17, 0.88; P = 0.18). In subgroup analysis, post-intervention endothelial function was significantly improved by antioxidant vitamin supplementation in T2DM subgroups with body mass index (BMI) ≤ 29.45 kg m−2 (SMD = 1.02; P < 0.05), but not in T2DM subgroups with BMI > 29.45 kg m−2 (SMD = −0.07; P = 0.70). In meta-regression, an inverse association was found between BMI and post-intervention SMD in endothelial function (B = −0.024, P = 0.02). Prolonged antioxidant vitamin E and/or C supplementation could be effective to improve endothelial function in non-obese T2DM subjects.