Background: Diabetes mellitus (DM) is reported to be related to pancreatic ductal adenocarcinoma (PDAC). Long-standing DM is a risk factor for PDAC, meanwhile, quite a few patients with PDAC develop DM as a paraneoplastic disorder and some papers reported that DM affected prognosis for PDAC. In this study, we investigated pre- and post-operative glucose tolerance in patients with pancreatectomy for PDAC or other disease. Methods: This single-center prospective study included 69 patients with pancreatectomy (40 pancreaticoduodenectomy (PD) and 29 distal pancreatectomy (DP) ) who received 75-g oral glucose tolerance test (OGTT) and glucagon test pre- and one month postoperatively. Plasma glucose, insulin, and C-peptide (CPR) at 0-, 30-, 60-, and 120-min during OGTT; and 0- and 6-min during glucagon test were obtained. These data and survival outcomes were analyzed. Results: There were 20 (29%) PDAC patients: 12 (30%) in PD group and eight (28%) in DP group. Nine patients with PDAC (45%) and seven patients (18%) without PDAC demonstrated DM type in preoperative OGTT. After pancreatectomy, 11 patients (55%) with PDAC and seven patients (15%) without PDAC experienced improvement in OGTT (P=0.0005). Greater improvement in homeostasis model assessment insulin resistance that were obtained by OGTT and used to measure insulin resistance, was noted after surgery in PDAC patients compared with non-PDAC patients (-1.4 vs -0.5 in PD group, P=0.07; -0.8 vs +0.06 P=0.05 in DP group). Delta CPRs obtained by glucagon test were significantly decreased postoperatively (3.0 to 1.1 ng/mL, P<0.0001 in PD group; 3.3 to 1.8 ng/mL, P<0.0001 in DP group). In survival analysis, fasting plasma glucose >110 mg/dL (HR 3.9, 95%CI 1.5-10, P=0.005) and the average of plasma insulin > 25 µIU/mL during OGTT (HR 0.36, 95%CI 0.14-0.93, P=0.035) were significant prognostic factor for PDAC. Conclusions: Pancreatectomy impaired insulin secretion and improve insulin resistance especially in PDAC patients. About half of PDAC patients demonstrated the improvement of glucose tolerance after surgery. Of note, glucose tolerance differed between PDAC and other disease, and affect the survival outcome for PDAC patients.