Background: Patients with Esophageal squamous cell carcinoma (EC) often present with advanced and metastatic stage disease, which has poor prognosis and limited treatment options, and represent an important unmet medical need especially in China/Asia. JS001, a humanized recombinant IgG4 antibody against PD-1, selectively blocks the interactions of PD-1 with its ligands PD-L1 and PD-L2, and promotes antigen specific T-cell activation. A phase I study of JS001 in Chinese patients with heavily pretreated solid tumors has demonstrated an acceptable safety profile in doses up to 10 mg/kg Q2W. Here we report the safety and efficacy of JS001 in a phase Ib/II clinical study in Chinese patients with refractory/metastatic EC. (Clinical trial ID: NCT02915432) Methods: Refractory/metastatic EC Patients receive JS001 3 mg/kg Q2W until disease progression or unacceptable toxicity. All patients with measurable disease will be assessed for clinical response every 8 weeks according to the RECIST 1.1 criteria. Tumor PD-L1 expression as well as additional potential predictive biomarkers are monitored for correlation with clinical response. Results: Between Apr 19 2017 and Sep 11 2017, 56 EC pts (84.9% received at least 2 Lines of treatment) were enrolled into the study. Treatment related adverse events occurred were mostly grade 1 or 2. As of Sep 11 2017, 34 EC pts have been evaluated for clinical efficacy. Among these pts, 8 PR (partial response) and 14 SD (stable disease) were observed (ORR 23.5% and DCR 64.7%). The PD-L1 expression positivity (defined as positive staining ³1% on Tumor Cell [TC] or on Immune Cell [IC] by SP142) in EC tumor tissue was 21.4% (12/56). 2/10 (20%) PD-L1+ patients and 6/24 (25%) PD-L1 negative patients achieved partial responses. Conclusions: JS001 showed a promising preliminary clinical activity in heavily pre-treated metastatic EC pts with a manageable safety profile. Clinical response was not correlated with PD-L1 expression. Pts will be continuously monitored for safety and efficacy upon JS001 treatment. Clinical trial information: NCT02915432