Aim Toexaminethecombinedeffectsof 5-ALA and metformin on diabetes mellitus induced by streptozotocin( STZ) in mice, and to discuss the mech-anisminitially.Methods Thediabeticmiceinduced by intraperitoneal injection of STZ were treated with 5-ALA and/or metformin for 30 days. Physical signs, fasting blood glucose ( FBG ) , fasting plasma insulin ( FIns) , plasma leptin ( Lep) , glucose tolerance, and histological changes of liver and pancreas were as-sessed. Insulin resistance was evaluated by the homeo-stasismodelassessment(HOMA).Results There were more significant effects when 5-ALA combined with metformin than only using metformin on lowering FBG, FIns and Lep. And 5-ALA combined with met-formin could improve glucose tolerance and the insulin sensitivity. Microscopic analysis demonstrated that sig-nificant changes in pancreatic islet and characteristic feature of the vacuolization phenotype in liver was ob-served on diabetic mice, and use of 5-ALA could in-hibit shrinking pancreatic islet number and hepatic morphological changes, especially in combination with metformin.Conclusions Asynergisticeffectof5-ALA and metformin is observed. Combined therapy of 5-ALA and metformin can decrease FBG and improve glucose tolerance, insulin sensitivity and islet ? cells function and morphology.