Abstract: To investigate the effects of metformin on pancreatic β-cell function and its possible mechanism, high fat diet-induced type 2 diabetic C57BL/6J mice were divided into two groups according to fasting blood glucose (FBG), glucose decreasing rate at 40 min of insulin tolerance test, triglycerides (TG), cholesterol (CHO) and body weight (BW). The C57 mice were gavaged with water or metformin for 58 days. β-Cell function was evaluated by oral glucose tolerance test and hyperglycemic clamp. Genes and proteins related to pancreas proliferation, lipid metabolism and endoplasmic reticulum stress were investigated. Compared with the model group, metformin group exhibited a reduction in the body weight (P<0.01), plasma TG and CHO (P<0.05), and the area under the curve (AUC) (P<0.05) of glucose tolerance test. The glucose infusion rate during clamp was improved (P<0.05) and the fasting insulin level was decreased in the metformin group (P<0.05). Metformin significantly upregulated the gene expression of pancreatic and duodenal homeobox 1 (Pdx-1, P<0.01) and liver X receptor β (Lxr-β, P<0.01). Western blot results showed that, the protein expression of PDX-1 was significantly upregulated (P<0.01). Endoplasmic reticulum stress related protein of activating transcription factor 4 (ATF4, P<0.001) and C/EBP homologous protein (CHOP, P<0.05) were also down-regulated. These results suggest that metformin could improve the insulin secretion function of type 2 diabetic C57BL/6J mice. The mechanism of the action may rely on its improvement of pancreas cell proliferation, lipid metabolism and amelioration of endoplasmic reticulum stress.