The objective was to explore the effects of metformin on the expression of endometrial glucose transporter 4 (GLUT4) and analyze the related factors of GLUT4 in patients with polycystic ovary syndrome (PCOS). This study included 20 obese patients with PCOS (PCOS group) and 20 obese patients who had infertility caused by oviducal or pelvic factors but had no PCOS (control group). Follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol-2 (E₂), testosterone (T), fasting serum glucose (FSG), fasting insulin serum (FINS), homeostasis model assessment-insulin resistance (HOMA-IR), and endometrial GLUT4 expression were determined in the two groups. In PCOS group, patients were given 500 mg of metformin three times per day for 3 mo, and then the parameters above were determined again and compared with that before metformin treatment. The parameters above also were compared between PCOS and control groups. The correlation of GLUT4 with its related factors was analyzed. The levels of T, FINS, and HOMA-IR were higher in PCOS group than in the control group (P < 0.01). The levels of protein and mRNA of endometrial GLUT4 were lower in the PCOS group than in the control group (P < 0.001). The expression of protein and mRNA of endometrial GLUT4 increased after metformin treatment (P < 0.001). HOMA-IR was negatively correlated with GLUT4 expression (P = 0.027). In patients with PCOS, the levels of protein and mRNA of endometrial GLUT4 were lower compared with that in non-PCOS women, and HOMA-IR was strongly associated with endometrial GLUT4 expression. Metformin may up-regulate endometrial GLUT4 expression to improve endometrial IR.