Objective: To investigated the role of intraoperative iodine-125 (125I) brachytherapy as a treatment option for advanced thoracic esophageal squamous cell carcinoma (ESCC). Methods: Using preoperative computed tomography (CT)-based staging criteria, between 2000 and 2008, 298 patients with ESCC (stage II-III) were enrolled in this prospective study. With informed consent, patients were randomized into two groups: intraoperative 125I seed implantation and surgery alone (control group). Twenty to forty 125I seeds (0.5 mCi per seed), with a total activity in 10~30 mCi, and a matched peripheral dose (MPD) of 60~70 Gy, were implanted under direct visualization. The surgical procedure used in this study was either a radical resection, which involved an esophagectomy through a left thoracotomy with two-field lymphadenectomy, or palliative resection. The postoperative complications were observed and recorded. The location and quality assessment of 125I seeds were assessed using CT scans or X-ray imaging. The short-term efficacy was evaluated according to WHO criteria. The 1, 3, 5 and 7-year survival rates were determined on follow-up. Results: There was no displacement or loss of 125I seeds. The local recurrence rates in the intraoperative 125I seed implantation group and control group were 14.9% and 38.7%, respectively (P < 0.05). An objective response rate of 92% was observed in the seed implant group, which was significantly higher than 0% in the control group (P < 0.05). There was no significant difference between the two groups when comparing of complications (P > 0.05). The 1-year survival rate of the two groups were not significantly different (P > 0.05). However, the 3, 5 and 7-year survival rates in the united 125I group (64%, 55.3% and 8%, respectively) were statistically different from those in the control group (52%, 29.1% and 1.4%,respectively)(P < 0.05). Conclusion: Intraoperative 125I seed implantation is safe and effective for advanced ESCC. Seed implantation may reduce the local recurrence rate and improve survival in patients with ESCC. The MPD of 60~70 Gy, with single 125I seed activity of 0.5 mCi, is reasonable.