Background and purpose: The purpose of this study is to evaluate the clinical impact of preoperative nutritional support for malnourished cancer patients. Methods: A total of 90 patients, 45 patients with nutrition support group and 45 patients with well-nourish group, in anticipating major surgery of pancreato-biliary cancer were enrolled in this study. We evaluated nutrition status by weight loss (>10% within 6 months), Patient-Generated Subjective Global Assessment (PG-SGA) grade B or C, serum albumin level (<3.0 g/dl) and assigned malnourished patients to the nutrition support group. Results: The postoperative complication rate in the nutrition support group was 51.1% and 42.2% in well-nourish group. The postoperative complications and day of hospital stay after operation were not different between each group. Although the patients whose nutritional status improved after nutritional support showed lower complication rate than other patients whose nutritional status were not improved, the complications rate in these patients were inferior to well-nourished patients. postoperative length of stay of subgroup (PG-SGA grade B/C à B/C) was marginally longer than that of well-nourished group (21.0 vs. 15.0 days, p = 0.050). Initial body mass index (OR, 1.49; 95% CI, 1.07–2.07; p = 0.020) and the amount of protein supplied (OR, 1.84; 95% CI 1.21–2.80; p= 0.004) were independently inhibit the nutritional improvement in malnutrition group. Conclusion: It is suggested that preoperative nutritional support in malnutritional pancreato-biliary cancer patients may be helpful to improve the clinical outcomes, but the postoperative results were still inferior to well-nourished patients in clinical implications.