Background: ctDNA is an attractive alternative to tissue for its easy accessibility and real time representation of systemic tumor profile. We explored the utility of ctDNA profiling during the course of mCRC treatment. Methods: Serial blood samples were obtained from mCRC patients before and during first-line palliative chemotherapy at fixed intervals (after every four cycles) until confirmed disease progression. ctDNA was sequenced using targeted next-generation sequencing (NGS) platform with 106 genes. Changes of ctDNA profile and treatment outcome were comprehensively analyzed. Results: A total of 272 samples from 62 patients were analyzed. In the pre-treatment blood samples, 56 (90.3%) of patients had detectable ctDNA mutations including single nucleotide variants, short insertions/deletions and copy number changes (median 4.5 mutations/ patient, range 0 - 133). In 31 (50.0%) patients who had tissue NGS panel results performed in the clinic, overall concordance between mutations from ctDNA and tissue was 86.5%. In three patients, ctDNA mutational profiles were found to be completely different from tissue profiles. At further investigation, these patients were found to have a separate primary cancer in their colon. At the time of the first follow-up, most (98.0%) patients showed decrease of ctDNA from baseline, represented by average variant allele frequency (VAF) changes of all ctDNA mutations found. Clearance of ctDNA was achieved in 40 (78.4%) patients and was associated with longer progression-free survival (median PFS 11.8 moths in ctDNA clearance (+) vs. 4.7 months in ctDNA clearance (-), p < 0.001). The ctDNA clearance at the same time point was able to further discriminate the patients in same category by RECIST 1.1. Serial follow-up monitoring revealed three patterns of ctDNA changes at the time of clinical progressive disease (PD): 1) re-emergence or re-increase of baseline ctDNA mutations, 2) emergence of new resistance mutations, 3) radiologic PD without evidence of ctDNA progression. In the patients with detectable ctDNA at PD, the ctDNA progression preceded radiologic progression in 25 (58.1%) patients by median of 3.3 months. The patients in clinical PD without ctDNA progression showed different patterns of metastasis having mainly extrahepatic spread, while 77.8% of the patients with ctDNA progression had their progression confirmed in liver metastasis. Diverse resistant mutations and gene amplifications in PD patients were discovered by ctDNA sequencing. For seven (16.3%) of the PD patients, the newly identified mutations could be potential candidates of targeted therapy or clinical trial. Conclusions: ctDNA profile provided additional information to conventional evaluation methods and reflected dynamic changes. ctDNA monitoring may improve precise treatment decision-making for individual patients.