Pancreatic cancer represents the most lethal of the common malignancies with a 5-year survival rate of less than 5%. For patients who are eligible for potentially curative resection, despite mortality and morbidity rates after surgery have improved, the recurrence rate is up to 85% within 2 years. Data from clinical trials indicate that adjuvant chemotherapy enhances 5-year survival to ~25% for patients who have undergone surgery to remove their tumor; and gemcitabine is the standard regimen of chemotherapy. Metformin is the first-line treatment for type 2 diabetes mellitus. Literatures reported that metformin might inhibit tumor growth by blocking some enzymes needed for cell growth. Some retrospective studies have revealed that diabetic patients taking metformin were less likely to develop pancreatic cancer. Additionally, pancreatic cancer patients treated with metformin showed a better survival than those without metformin. In this study, the researchers intend to investigate the activity and safety of the combination of gemcitabine and metformin in treating patients with pancreatic cancer that have removed by surgery.