Abstract: Roux-en-Y gastric bypass surgery (RYGB) has been demonstrated to be successful for treating type-II diabetes mellitus (T2DM) patients with a body mass index (BMI) ,30 kg/m2, but reports of RYGB for T2DM patients with a BMI ,28 kg/m2 are lacking. T2DM patients with a BMI ,28 kg/m2 were prospectively recruited to participate in this study in four hospitals. The endpoint was T2DM remission (defined by fasting blood glucose (FBG) level ,110 mg/dL and hemoglobin (Hb)A1c level ,6.0% at 12 months postoperatively). Predictors of remission were investigated by univariate and multivariate analyses. Eighty-six patients were assessed. Eighty-five patients underwent RYGB, with one conversion to open surgery. We compared the values of various variables before and after surgery. The mean BMI decreased from 24.68?2.12 to 21.72?2.43 kg/m2 (P,0.001). Fifty-eight (67.4%) patients were not treated by drugs or insulin after surgery, and 20 patients (23.3%) had complete remission of T2DM at 12 months after surgery with an acceptable number of complications. The mean HbA1c level in the remission group was significantly lower than that in the non-remission group. Patients with a higher weight, lower HbA1c level, higher C-peptide level, and higher FBG level were more likely to have T2DM remission in multivariate analyses. In conclusion, RYGB was effective and safe for treating T2DM patients with a BMI ,28 kg/m2. Complete remission can be predicted by cases having a higher weight, lower HbA1c level, higher C-peptide level, and higher FBG level.