Objectives To assess the efficacy and safety of metformin plus rosiglitazone in treating type 2 diabetes mellitus. Methods Based on the principles and methods of Cochrane systematic reviews, we searched the CochraneLibrary (2008, 4 issue), PubMed (1966 to October 19, 2008), Embase (1974 to October 19, 2008), China BiomedicalLiterature Database (1978 to October 12, 2008), China Journal Fulltext Database (1994 to October 12, 2008), ChineseScientific Journals Full text Database (1989 to October 12, 2008). Randomized controlled trials (RCTs) of Metforminplus roziglitazone versus metformin for type 2 diabetes were included. We assessed the quality of the included RCTsaccording to the Cochrane Handbook for Systematic Reviews of Interventions Version 5.0.1. The Cochrane Collaboration’s software RevMan 5.0 was used for meta-analysis. Results Twelve RCTs totaling 3020 patients were included. Metaanalysis showed that Glycosylated hemoglobin levels [WMD= – 0.48%, 95%CI (– 0.74, – 0.22), P=0.000 3], fasting plasma glucose levels [WMD= – 1.03mmol/L, 95%CI (– 1.85, – 0.75), Plt;0.000 01], insulin sensitivity, and β-cell function improved significantly with metformin plus rosiglitazone therapy. Compared with the metformin monotherapy group, patients treated with metformin plus rosiglitazone had more edema events [RR= 3.27, 95%CI (1.80, 5.91), Plt;0.000 1] and lower gastro-intestinal events [RR= 0.82, 95%CI (0.71, 0.94), P=0.004]. We found no statistically significant effect on body weight, the percentage of patients with at least one adverse event, and hypoglycemia events. Conclusions Current evidence demonstrates that combination treatment with metformin plus rosiglitazone improves glycemic control, insulin sensitivity, and cells function more effectively than with metformin monotherapy. Side effects of two types of therapy have differences in performance.