Objective To determine the maximum-tolerated dose (MTD) and dose-limiting toxicities (DLTs) of capecitabine during postoperative concurrent capecitabine and intensity-modulated radiotherapy (IMRT) for locally advanced gastric cancer. Methods The patients, who had T_xN (+) M₀ gastroesophageal or gastric adenocarcinoma after radical surgery with negative margins (R₀) or non-radical surgery with microscopically positive margins (R₁) or macroscopically positive margins (R₂), were included in the study. A total of 18 patients (R₀ 12;R₁ 2;R₂ 4) were recruited. IMRT (45 Gy at 1.8 Gy/fraction) was delivered to the tumor bed (T_4b only), anastomosis site, duodenal stump, and regional lymph nodes days per week for 5 weeks. Patients with R₁ or R₂ resection received additional radiation (10.8 Gy/6 fractions) to the microscopically or macroscopically positive margins. During IMRT, capecitabine was orally administered twice daily at a dose of 625 mg/m²(level I, n=6), 700 mg/m²(level Ⅱ, n=6), 800 mg/m²(level Ⅲ, n=6), 900 mg/m²(level IV, n=0), and 1000 mg/m²(level V, n=0). DLT was defined as grade 3 or 4 hematologic and non-hematologic toxicities. Results Grade 1—3 leukopenia (89%), anorexia (83%), and nausea (83%) were the most common toxicities. Grade 3 anorexia/nausea and grade 4 vomiting occurred in one patient at dose level I. Grade 3 anorexia and nausea occurred in one patient at dose level Ⅱ. One patient at dose level Ⅲ developed grade 4 neutropenia, while another patient at dose level Ⅲ developed grade 3 radiation esophagitis. Conclusions During postoperative concurrent capecitabine and IMRT for locally advanced gastric cancer, the MTD of capecitabine is 800 mg/m² twice daily, and the DLTs are nausea, vomiting, anorexia, neutropenia, and radiation esophagitis.