Esophageal squamous cell carcinoma is a common malignancy in China. Although neoadjuvant chemoradiotherapy followed by esophagectomy remains a standard modality for locally advanced esophageal squamous cell carcinoma, esophagectomy followed by postoperative radiotherapy is also prevalent in China. Several retrospective studies demonstrated that postoperative radiotherapy could improve the prognosis of patients. Nevertheless, there still existed approximately 11.5% and 17.2% of total patients developing local-regional relapse and hematological metastasis. The result of Checkmate 577 has shown that postoperative immunotherapy of nivolumab could improve the disease-free survival (median Disease-free Survival 29.7 mos vs. 11.0 mos). Therefore, investigators aimed to implement a pilot study to explore the safety and efficacy of combining postoperative radiotherapy and immunotherapy for patients with locally advanced esophageal squamous cell carcinoma after esophagectomy.