Objective To evaluate the efficacy of laparoscopic Roux-en-Y gastric bypass(LRYGB)in the treatment of morbid obesity complicated with type 2 diabetes mellitus,and to discuss the mechanism of LRYGB in the treatment of type 2 diabetes mellitus.Methods The clinical data of 30 type 2 diabetes mellitus patients with body mass index(BMI)≥35.00 who had undergone LRYGB from January 2007 to July 2007 in Cleveland Clinic were prospectively analyzed.The pre-and postoperative clinical parameters wcl"e analyzed by t test.Results The BMI,fasting plasma glucose and glycosylated hemoglobin were significantly decreased 6 months after the operation(t=27.399,23.025.15.593,P＜0.05).The cure rate at the end ofthe second month was 70%(21/30),and it was increased to 90%at the end of sixth month(27/30).The condition of the remaining 3 patients were improved.At the end of the second and sixth month',ffter operation,the BMIs of the patients who were cured by LRYGB were decreased by 27.89%±5.51%and 45.73%±2.82%.and the BMIs of the patients whose condition was improved were decreased by 35.65%±1.97%and 58.00%±3.05%(t=5.755,7.081,P＜0.05).Conclusions LRYGB is effective in the treatment of morbid obesity complicated with type 2 diabetes mellitus.The curability of morbid obesity increases as the BMI decreases.