Background: Expanded RAS analysis is essential for the selection of biologic agents in mCRC. RAS mutations indicates anti-EGFR unresponsiveness. In this study, we aimed to investigate RAS and BRAF mutations by liquid biopsy at progression in patients with RAS mutant mCRC. Methods: Sixty patients with mCRC who harbored tissue RAS mutations were prospectively analyzed between July 2019 and April 2020. All the patients treated with chemotherapy plus bevacizumab combinations . The plasma samples of the patients were analyzed after progression of bevacizumab combinations. RAS mutation profile was evaluated in plasma using Idylla PCR-based molecular diagnostics method, which enables rapid detection of common mutations in RAS and BRAF genes in circulating tumor DNA (ctDNA). Kaplan-Meier method was used for survival analysis and log-rank test was performed for comparison of groups. Results: The median age of the patients was 60 years (IQR:35-83 years) and female was (n=23, 38.3%). Primary tumor was located in the left colon in 81.7% of all patients. There were 95.0% KRAS and 5% NRAS mutations in baseline tissue biopsy. As a result of liquid biopsy after progression, 55.0% of the patients had KRAS, 3.3% NRAS and 3.3% had BRAF mutations. The RAS mutation detected in 58.3% of the patients. While there was no significant difference in terms of clinicopathological features between wild type (RAS/BRAF) and mutant type (RAS/BRAF) determined by liquid biopsy, the overall survival (OS) of the wild type group was significantly longer than mutant group (43.8 vs. 20.4 months, p= 0.002). Conclusions: This study demonstrated that there may be changes in RAS/BRAF mutation from plasma analysis after progression in patients with mCRC. Since better survival in the patient group with wild type was detected compared to the RAS concordance group, the evaluation of RAS mutation status at the time of progression may be important in terms of disease prognosis and treatment options.