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,N ( + ) 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 (T4b 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 m , 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 ]I. One patient at dose level ]~ 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 capeeitabine is 800 mg/m2 twice daily, and the DLTs are nausea, vomiting, anorexia, neutropenia, and radiation esophagitis. Key words: Capecitabine; Maximum tolerated dose; Dose-limiting toxicity; Gastric neoplasms/concurrent chemoradiotherapy; Radiotherapy, intensity-modulated