Roux-en-Y gastric bypass (RYGB) is a commonly used surgical treatment for patients with morbid obesity. It significantly and persistently decreases the levels of blood glucose and glycosylated hemoglobin in 80-100 percent of type 2 diabetes mellitus (T2DM) patients. A study group from China found the possible mechanism of treating T2DM with RYGB surgery. This study has provided new basis for surgery to treat T2DM and explained why the post-surgical nesidioblastosis occurs in RYGB patients.