In this study, the investigators will enroll approximately 320 mCRC patients (It was considered that an increase of response rate of 15% compared to conventional irinotecan dose of 180 mg/m2, and these were chosen as parameters with which to calculate the study power. Initial power calculation was suggested that a minimum of 140 patients in each group would be required to achieve statistical significance with a power of 80% at the 5% significance level. It is estimated that about 10% of 320 mCRC patients fail to complete the study). For these enrolled patients, the investigators will randomize and divide these patients into two groups: control group and study group. Control group includes mCRC patients who will receive the conventional regimen of FOLFIRI plus bevacizumab. Otherwise, patients in the study group will have genotyping of UGT1A1 before therapy, and dose escalating of irinotecan will depend on results of genotyping.