Background: Over the past few decades, bariatric surgery, especially Roux-en-Y gastric bypass (RYGB), has become widely considered the most effective treatment for morbid obesity. In most cases, it results in enhanced glucose management in patients with obesity and type 2 diabetes (T2D), which is observed before significant weight loss. However, what accounts for this effect remains controversial. To gain insight into the benefits of RYGB in T2D, we investigated changes in the beta-cell mass of obese rats following RYGB. Material/Methods: RYGB or a sham operation was performed on obese rats that had been fed a high-fat diet (HFD) for 16 weeks. Then, the HFD was continued for 8 weeks in both groups. Additional normal chow diet (NCD) and obese groups were used as controls. Results: In the present study, RYGB induced improved glycemic control and enhanced beta-cell function, which was reflected in a better glucose tolerance and a rapidly increased secretion of insulin and C-peptide after glucose administration. Consistently, rats in the RYGB group displayed increased beta-cell mass and islet numbers, which were attributed in part to increased glucagon-like peptide 1 levels following RYGB. Conclusions: Our data indicate that RYGB can improve beta-cell function via increasing beta-cell mass, which plays a key role in improved glycemic control after RYGB.