4038 Background: The most commonly used treatment methods for metastatic colorectal cancer (mCRC)are systemic chemotherapy, molecular targeted therapy and local treatment. The main chemotherapy drugs for mCRC include Irinotecan, Oxaliplatin and 5-Fu. V308 Research shows that FOLFOX and FOLFIRI can be standard first or second-line of each other in the treatment of metastatic colorectal cancer. However if the first-line treatment regimen containing 5-FU fails, whether it is necessary to re-challenge 5-FU when Irinotecan is applied in the second line is unknown. There is no head-to-head comparative study to answer whether the FOLFIRI regimen is better than the Irinotecan monodrug. Therefore, it is necessary to carry out a comparative study of FOLFIRI Versus Irinotecan monodrug to observe whether adding 5-Fu on the basis of Irinotecan can improve the therapeutic effect. Methods: This was a randomized phase III trial. Patients from 5 centers in China with metastatic colorectal adenocarcinoma, for whom first-line of chemotherapy including oxaliplatin combined with fluorouracil drugs (combined or not combined with targeted therapy) had failed, were enrolled. 172 patients with mCRC were randomly treated with FOLFIRI or Irinotecan monodrug were included in this study. FOLFIRI group: Irinotecan 180mg/m²; Lecovorin 400mg/m²; 5-Fu 400mg/m²; 5-Fu 2400mg/m² CIV 46h. Irinotecan monodrug group 180mg/m², The regimen was repeated every 2 weeks. The primary endpoint is PFS, and this clinical trail is a superiority trial. Results: ITT (Intention-To-Treat) analysis: Among 172 patients, 10 had PR, 93 had SD, and 63 had PD, 6 patients have not received efficacy evaluation yet. The ORR was 5.68% VS. 5.95%, and the DCR was 61.36% and 54.76% in FOLFIRI group and Irinotecan monodrug group, respectively. Adverse reactions included neutropenia, stomatitis, diarrhea, fatigue, abnormal liver enzymes, pyrexia, arrhythmia, nausea and most of these were grade 1-2. The dose reduction rate induced by drug tocixity of was 13.64% and 7.14% in FOLFIRI group and Irinotecan monodrug group, respectively. Conclusions: These data show that Irinotecan monodrug has the similar ORR and DCR with FOLFIRI regimen in second-line treatment of mCRC. Irinotecan monodrug has lower adverse effect. Clinical trial information: NCT02935764 .