The potential therapeutic effects of metformin on several cancerswere reported. However, the evidence of the effects of metformin on ovarian cancer is still limited and inconclusive. This systematic review and meta-analysis study aims to summarize the existing evidence of the therapeutic effects of metformin on ovarian cancer. Methods: We performed systematic searches using electronic databases including PubMed and EMBASE until December 2012. Key words included metformin AND ( ovarian cancer OR ovary tumor ). All human studies assessing the effects of metformin on ovarian cancer were eligible for inclusion. All articles were reviewed independently by 2 authors with a standardized approach for the purpose of study, study design, patient characteristics, exposure, and outcomes. The data were pooled using a random-effects model. Results: Of 190 studies retrieved, only 3 observational studies and 1 report of 2 randomized controlled trials were included. Among those studies, 2 reported the effects of metformin on survival outcomes of ovarian cancer, whereas the other 2 reported the effects of metformin on ovarian cancer prevention. The findings of studies reporting the effects on survival outcomes indicated that metformin may prolong overall, disease-specific, and progression-free survival in ovarian cancer patients. The results of studies reporting the effects of metformin on ovarian cancer prevention were meta-analyzed. It indicated that metformin tended to decrease occurrence of ovarian cancer among diabetic patients with the pooled odds ratio of 0.57 (95 confidence interval, 0.16-1.99). Conclusions: Our findings showed the potential therapeutic effects of metformin on survival outcomes of ovarian cancer and ovarian cancer prevention. However, most of the evidence was observational studies. There is a call for further well-conducted controlled clinical trials to confirm the effects of metformin on ovarian cancer survival and ovarian cancer prevention.