ObjectiveTo systematically evaluate the effects of vitamin D supplementation on fasting blood glucose, insulin resistance, β cell function in type 2 diabetes mellitus. MethodsDatabases including PubMed, The Cochrane Library (Issue 12, 2015), Web of Science, ScienceDirect Online, VIP, CNKI, WanFang Data, and CBM were searched to collect randomized controlled trials (RCTs) about vitamin D supplementation for type 2 diabetes mellitus from inception to December 2015. Two reviewers independently screened literature, extracted data and assessed the risk of bias of included studies. Then meta-analysis was conducted by RevMan 5.3 and Stata12.0 softwares. ResultsA total of 22 RCTs involving 1 756 patients were included. The results of meta-analysis showed that, compared with the control group, the vitamin D supplementation group had a significant improvement in insulin resistance (SMD=–0.68, 95%CI –1.23 to –0.12, P=0.02), but there were no significant differences in levels of FPG, HbA1c and HOMA-β between the two groups (all P value > 0.05). Subgroup analysis showed that, the levels of FPG and HOMA-IR were significantly decreased in the vitamin D supplementation group in Middle Easterners and patients whose follow-up duration was less than three months. ConclusionVitamin D supplementation could improve HOMA-IR but could not improve the levels of FPG, HbA1c and HOMA-β. However, the evidence is weak to recommend vitamin D as a means of improving glycemic control, insulin resistance and β cell function in type 2 diabetes mellitus. Further larger, high quality trials are warranted.