Background: In locally advanced or metastatic biliary tract cancer (BTC), second-line chemotherapy is challenging after progression from first-line gemcitabine/cisplatin, although mFOLFOX has been proven to be superior to active symptom control in ABC-06 trial. Irinotecan is an active drug in other gastrointestinal cancers. This study evaluated whether mFOLFIRI was superior to mFOLFOX in second-line treatment of BTC. Methods: Patients diagnosed with BTC with disease progression after prior gemcitabine/cisplatin were randomized (1:1) to either mFOLFOX (oxaliplatin 100mg/m2 over 2 hours, leucovorin 100mg/m2 over 2 hours, 5-fluorouracil 2400mg/m2 over 46 hours, every 2 weeks) or mFOLFIRI (irinotecan 150mg/m2 over 2 hours, leucovorin 100mg/m2 over 2 hours, 5-fluorouracil 2400mg/m2 over 46 hours). Randomization was stratified by tumor location (intrahepatic vs extrahepatic vs gallbladder vs ampulla of vater) and ECOG performance status (0, 1 vs 2). Primary end-point was overall survival (OS) rate at 6 months. Results: In total, 120 patients were enrolled and 114 patients were treated (mFOLFOX:57, mFOLFIRI:57). Median age was 63 years old. Most patients had ECOG 0/1 (89.5%). Tumor location was intrahepatic in 47 patients (41.2%), extrahepatic in 27 (23.7%), gallbladder in 35 (30.7%) and ampulla of vater in 5 (4.4%). At the median follow-up duration of 10.7 months (95% CI, 8.2-13.2), 6-month OS rate was 58.1% in mFOLFOX and 46.0% in mFOLFIRI. Of 102 evaluable patients (mFOLFOX:51, mFOFIRI:51), objective response rate and disease control rate were 5.9% (95% CI, 0-12.4) and 64.7% (95% CI, 51.6-77.8) in mFOLFOX and 3.9% (95% CI, 0-9.2) and 58.8% (95% CI, 45.3-72.3) in mFOLFIRI. Median progression-free survival was 2.8 months (95% CI, 2.3-3.3) in mFOLFOX and 2.1 months (95% CI, 1.3-2.9) in mFOLFIRI (p = 0.682). Median OS was 6.6 months (95% CI, 5.6-7.6) in mFOLFOX and 5.9 months (95% CI, 4.3-7.5) in mFOLFIRI (p = 0.887). The most common grade 3/4 adverse events were neutropenia (26.3%) and AST/ALT elevation (15.8%) in mFOLFOX and neutropenia (24.6%) and anemia (17.5%) in mFOLFIRI. Peripheral neuropathy (36.8%) and thrombocytopenia (35.1%) in mFOLFOX and vomiting (19.3%) and cholangitis (10.5%) in mFOLFIRI occurred more frequently. No chemotherapy-related deaths were reported. Conclusions: In second-line treatment of BTC, mFOLFIRI was tolerable But, mFOLFIRI was not superior to mFOLFOX. Adverse events were different between two groups. Clinical trial information: NCT03464968.