Background: Anti-Epidermal growth factor receptor (EGFR) antibody therapy is expected to be effective in treatment for metastatic colorectal cancer (mCRC) with wild-type KRAS, but for mCRC with mutated KRAS, no salvage treatment has been established. We performed a phase II clinical study on 3rd-line chemotherapy combined bevacizumab with S-1, an oral fluorinated pyrimidine preparation containing a dihydropyrimidine dehydrogenase inhibitor, and bevacizumab for mCRC resistant to oxaliplatin and irinotecan. Methods: Subjects were mCRC patients with mutated KRAS, who showed aggravation even after 2 regimens with oxaliplatin and irinotecan. S-1 (80-120 mg/body) was administered for 4 weeks and withdrawn for 2 weeks. The dose of S-1 was decided according to the subjects’ body surface area. Bevacizumab (5 mg/kg) was administered on Days 1, 15, and 29. This treatment was provided until progression. The primary endpoint was disease control rate (DCR), and secondary endpoints were response rate (RR), median progression free survival (mPFS), overall survival (OS), and adverse event (AE). Results: A total of 31 subjects mutated KRAS were enrolled between August 2009 and July 2011. An independent review committee evaluated antitumor effects in eligible 29 of the 31 subjects in accordance with the Response Evaluation Criteria in Solid Tumors (RECIST). Two subjects in whom antitumor effects could not be evaluated were excluded. The DCR was 69% (95% confidence interval [CI], 49.2-84.7%), RR 0% (95% CI, 0-12.3%), mPFS 3.7 months (95% CI, 2.7-6.5 months), OS 9.0 months (95% CI, 7.5-12.0 months), and the median observation period 9.0 months. In 30 subjects for safety evaluation, the incidence of Grade 3 or greater adverse events was 50%. There was no treatment-related death. Major adverse events were anorexia (Grade 3 or greater, 20%), diarrhea (Grade 3, 10%), and decreased hemoglobin (Grade 3 or greater, 16.7%). Conclusions: The results suggest that 3rd-line chemotherapy combined bevacizumab with S-1 is safe and may delay the progression of mCRC resistant to oxaliplatin and irinotecan with mutated KRAS. Clinical trial information: NCT00974389.