Background: The prognosis of patients with unresectable or metastatic biliary tract cancer (BTC) is unacceptable low. This study aimed to determine the efficacy, safety and predicting biomarkers of immune checkpoint inhibitor nivolumab in combination with chemotherapy in advanced BTCs.Methods: In this open-label, single-arm, phase Ⅱ trial, chemo-immunology combination therapy consisting of gemcitabine 1000mg/m2, cisplatin 75mg/m2 plus nivolumab 3mg/kg was administered every 3 weeks for up to 6 cycles. Maintenance treatment with gemcitabine plus nivolumab was administered to patients achieving disease control following the combination therapy. The primary outcome was objective response rate (ORR). Secondary outcomes included safety, disease control rate (DCR), progression free survival (PFS) and overall survival (OS). The exploratory objectives were to assess biomarkers for predicting clinical response and prognosis.Results: 32 patients with a median age of 60 (range 27-69) years were enrolled. As of September 31, 2019, the median follow-up was 12.8 (95% CI, 10.8-14.8) months. 27 response-evaluable patients received a median of 4 (IQR, 3-6) cycles combination therapy, of which 15 (55.6%) patients achieved objective response, including 5 (18.6%)with complete response (CR), and the DCR was 92.6%. 2 of 6 patients in cohort A who were refractory to gemcitabine or cisplatin-based chemotherapy achieved 1 CR and 1 partial response. 13 of 21 chemotherapy-naive patients (61.9%) in cohort B achieved objective response. The median PFS of all patients in cohort A+B was 6.1 months. The median OS was 8.5 months with 33.3% 12-month OS rate. The most frequent grade 3 or higher adverse events were thrombocytopenia (56%), and neutropenia (22%). Fitness might be a biomarker for predicting clinical response. On-therapy change of serum sFASL, MCP-1 and IFN-γ were correlated with prognosis.Conclusions: Nivolumab in combination with gemcitabine and cisplatin offer promising efficacy and manageable safety profile for patients with advanced BTCs.