Metformin is an antidiabetic drug used widely in clinical practice. Its main clinical effect is to reduce blood glucose levels by improving insulin resistance. Nonalcoholic fatty liver disease is characterized by chronic liver damage and can develop into liver cirrhosis. Nonalcoholic fatty liver disease is associated with obesity and contributes to insulin resistance, and metformin is used to treat individuals with these conditions. The mechanisms underlying the clinical effects of metformin in treating nonalcoholic fatty liver disease are unclear. This article summarizes the literature on the mechanisms associated with liver glucose metabolism and the beneficial effects of metformin on this common liver disease.