Objectives: Post-transplantation diabetes mellitus (PTDM) is an important risk factor for cardiovascular disease and mortality. There are few studies on the safety and effectiveness of metformin in kidney transplant recipients. We aimed to determine the influence of metformin on cardiovascular outcome in PTDM. Methods: We included 615 kidney transplant recipients with new-onset PTDM in Asan Medical Center and Dongguk University Ilsan Hospital. PTDM was defined as maintained hypoglycemic treatment, including oral hypoglycemic agents and insulin, three months or more after transplantation. Participants were classified as metformin group and non-metformin group. Results: During a mean follow-up of 10.2 years, 259 PTDM patients (42.1%) were prescribed metformin. The number of recipients who received coronary intervention was 3 (1.1%) in metformin group and 21 (5.9%) in non-metformin group. In Cox proportional hazard model, metformin usage was associated with lower risk of coronary arterial disease (hazard ratio [HR]: 0.27; 95% confidence interval [CI]: 0.08–0.94; p=0.04). In addition, compared with the non-metformin group, the HR of the long-term use of metformin (>1200 days) was 0.17 (95% CI 0.02–1.31; p=0.08), and the HR of the short-term use (<1200 days) was 0.38 (95% CI 0.08–1.65; p=0.19). Conclusions: This study demonstrates that the use of metformin was associated with a decreased risk of coronary arterial disease in kidney transplant recipients with PTDM. Metformin would be considered as one of feasible treatment options for PTDM.