Introduction: Metformin has been demonstrated to enhance cardioprotective benefits in type 1 diabetes(T1DM). Although glycemic variability (GV) is associated with increased risk of CVD in diabetes, there is a scarcity of research evaluating the effect of metformin on GV in T1DM. Objectives: In the present study, the effects of adjuvant metformin therapy on GV and metabolic control in T1DM were explored. Patients and methods: A total of 65 adults with T1DM were enrolled and subjected to physical examination, fasting laboratory tests and continuous glucose monitoring, and subsequently randomized 1:1 to 3 months of 1000- 2000 mg metformin daily add-on insulin (MET+INS, n=34) or insulin (INS, n=31). After, baseline measurements were repeated. Results: The mean amplitude of glycemic excursions was substantially reduced in the MET+INS group, compared with the INS group (-1.47±3.39 mmol/L versus 1.05±4.24 mmol/L, P=0.012). In parallel, the largest amplitude of glycemic excursions (-2.28±4.71 mmol/L versus 1.77±5.71 mmol/L, P=0.003), the standard deviation of blood glucose (- 0.62±1.15 mmol/L versus 0.08±1.23 mmol/L, P=0.023), and the coefficient of variation (-6.08±12.31 % versus 2.29±11.57 %, P=0.008) all demonstrated improvement in the MET+INS group, compared with the INS group. Significant reduction in the insulin dose, body mass index and body weight were observed in patients with MET+INS, not those with INS. Conclusion: Additional metformin therapy improved GV in adults with T1DM, as well as improving body composition and reducing insulin requirement. Hence, metformin as adjunctive therapy has potential prospects in reducing the CVD risk in patients with T1DM in the long term.