To evaluate the prognosis of type II diabetes mellitus (T2DM) after gastrectomy and related factors in gastric cancer patients. 403 gastric cancer patients with T2DM were studied, who underwent gastrectomy between May 2003 and September 2009. A review of medical records and telephone interviews was performed in this cross-sectional study. The factors included in the statistical analysis were as follows: gender, age, type of surgery, preoperative body mass index (BMI), current BMI, BMI reduction ratio, preoperative insulin or oral diabetic medicine requirement, follow-up duration, and current state of diabetes. Assessment of diabetes status after surgery was classified into four categories according to the change in hypoglycemic agents after surgery and present status of T2DM: resolution, improvement, same, and worse. The mean follow-up duration was 33.7 mo (± 20.6 mo), preoperative BMI was 24.7 kg/m(2) (± 3.0 kg/m(2)), and BMI reduction ratio was 9.8% (± 8.6%). After surgery, T2DM was cured in 58 patients (15.1%) and was improved in 117 patients (30.4%). According to the type of surgery, the BMI reduction ratio was significantly higher in the total gastrectomy and Roux-en-Y reconstruction group [14.2% ± 9.2% vs 9.2% ± 7.7% (Billroth II group), P < 0.001] and significantly lower in the subtotal gastrectomy and Billroth I reconstruction group [7.6% ± 8.0%, 9.2% ± 7.7% (Billroth II group), P < 0.001]. The BMI reduction ratio, follow-up duration after surgery, type of surgery, extent of gastrectomy, and performance of duodenal bypass were significantly correlated to the course of T2DM (P < 0.05). The BMI reduction ratio was the most influential factor on T2DM status. In a subgroup analysis of patients with a BMI reduction ratio of 10% or less (n = 206), T2DM was cured in 15 (7.6%) patients and was improved in 57 (28.8%) patients after surgery, and only the duration of surgery was significantly correlated to T2DM status (P = 0.022). The course of T2DM was significantly correlated to the BMI reduction ratio but not to the type of surgery without a significant change in BMI.