Background: Lean body mass loss after surgery, which decreases compliance of adjuvant chemotherapy, is frequently observed in gastric cancer patients who underwent gastrectomy for gastric cancer. However, the risk factors of lean body mass loss remain unclear. Methods: The present study retrospectively examined the patients who underwent curative gastrectomy for gastric cancer between June 2010 and March 2014 at Kanagawa Cancer Center. All patients received perioperative care of the enhanced recovery after surgery protocol. % Lean body mass loss was calculated by percentile of lean body mass at one month after surgery to preoperative lean body mass. Severe lean body mass loss was defined as % lean body mass loss over 5%. Risk factors for severe lean body mass loss were determined by both univariate and multivariate logistic regression analyses. Results: Four-hundred eighty five patients were examined. Median age was 67 years. Operative procedure was total gastrectomy in 190 patients and distal gastrectomy in 295 patients. Surgical complications of grade 2 or more defined by Clavien-Dindo classification was observed in 78 patients including pancreatic fistula in 19, anastomotic leakage in 11 and abdominal abscess in 7. Mortality was observed in one patient. Both univariate and multivariate logistic analyses demonstrated that surgical complications (odds rate 3.576, p = 0.001), total gastrectomy (odds rate 2.522, p = 0.0001), and gender (odds rate 1.928, p = 0.001) were significant independent risk factors for severe lean body mass loss. Conclusions: Male, surgical complications, and total gastrectomy were significant risk factors for 5% of lean body mass loss at first month after gastrectomy. To maintain lean body mass after gastrectomy, the physician need careful attention for the patients who had these risk factors.