We investigated the effect of metformin on hepatic glucose production and peripheral glucose uptake in Asian patients with type 2 diabetes mellitus. We recruited ten Japanese patients whose fasting glucose levels remained poorly controlled under meal-time injection of short-acting insulin. Metformin was added to their insulin therapy, and both hepatic glucose production and peripheral glucose uptake were assessed before and one week after metformin treatment, with the use of stable isotope [6,6-²H₂] glucose. Metformin was titrated to a maximum dose of 500 mg. As a result, fasting glucose levels and hepatic glucose production were significantly improved after the metformin treatment (p < 0.01 and 0.02), whereas their peripheral glucose uptake was not significantly changed (p = 0.63). Furthermore, the change of fasting glucose levels was significantly correlated with that of hepatic glucose production, whose coefficient ρ was 0.76 (p = 0.01). On the other hand, there was no significant correlation between the change of fasting glucose levels and that of peripheral glucose uptake (p = 0.43). In conclusion, low dose of metformin significantly reduced hepatic glucose production in Japanese patients with type 2 diabetes mellitus. The efficacy of metformin on correcting fasting hyperglycemia was strongly associated with reduced hepatic glucose production, rather than ameliorated peripheral glucose uptake.