Background: LA-ESCC is a potentially curable disease, for which preoperative CRT followed by esophagectomy is a standard-of-care. Previous studies have identified that close/involved margin and lymph node metastasis with extranodal invasion (ENI) in post-CRT surgical specimens are associated with increased risk of recurrence. In CheckMate-577 trial, adjuvant nivolumab significantly improved disease-free survival (DFS) in patients with esophageal cancer treated with preoperative CRT and surgery; in another trial (the “PACIFIC” trial), adjuvant durvalumab has significantly improved overall survival (OS) in stage III non-small cell lung cancer treated with definitive CRT. We hypothesize that adding pembrolizumab to CRT as an adjuvant therapy would improve the outcomes of LA-ESCC patients who are treated with preoperative CRT plus esophagectomy and with high-risk of recurrence. Methods: This single institution single-arm phase II study include patients with histologically confirmed LA-ESCC (AJCC 7th staging system:cT3-4aN0M0 or T1-4aN1-3M0) harboring at least one risk factor (closed/involved margin, ENI, or ypN2-3) in post-CRT surgical specimens. Patients with adenocarcinoma of esophagus or gastroesophageal junction or synchronously diagnosed with a squamous cell carcinoma of aero-digestive way other than ESCC are excluded. Patients enrolled will receive adjuvant weekly cisplatin–CRT (cisplatin, 30mg/m² for 2 cycles every week; radiotherapy, 180-200 cGy/fraction for 10-13 times) followed by pembrolizumab (200 mg, every 3 weeks, for 18 cycles). The primary endpoint of this study is 1-year relapse-free survival (RFS) rate; and the key secondary endpoints include RFS, 3-year RFS rate, OS, 3-yr OS rate, toxicity and safety. The study, registered with clinical trial ID of NCT03322267, started patient enrollment in Aug 2018. As of Aug of 2020, 11 of 46 planned patients have been enrolled. Clinical trial information: NCT03322267