Abstract: Objective To detect K-Ras gene mutations by direct sequencing and peptide nucleic acid clamp PCR assay（PNA-PCR），and analyze the correlation between K-Ras mutations and prognosis in metastatic colorectal cancer（mCRC）. MethodsA total of 110 paraffin-embedded mCRC tissues were collected. Direct sequencing and PNA-PCR methods were used to detect 12 and 13 codon mutations of K-Ras gene and analyze the correlations between K-Ras mutations and prognosis. Results Forty-three cases of K-Ras mutation were detected by direct sequencing method. But for the PNA-PCR method，not only these mutations were detected and another 10 cases of mutations were detected. The difference of median overall survival（OS） between wild-type and mutant-type detected by direct sequencing was close to statistical significance （20.5months vs. 15.6months，P=0.067）. And for PNAPCR method，the difference was significant（21.3 months vs. 15.8 months，P=0.014）. There was no difference of progress free survival（PFS）in both wild-type and mutant-type detected by the two methods. According to the detection of these two methods，110 cases were divided into 3 groups：the high mutation group，the low mutation group and the wild-type group. Only the OS between the wild-type group and low mutation group showed differences（15.6 months vs.21.3 months，P=0.040）. Cox regression model showed that ECOG score（HR=2.70，95％CI：1.39-5.25，P=0.003） and K-Ras status（HR=1.52，95％CI：1.52-2.19，P=0.026）were independent factors for prognosis. Conclusion K-Ras mutation is not a efficacy predicting factor for irinotecan or oxaliplatinbased therapies. The K-Ras mutations detected by PNA-PCR method have relation to the OS of patients. For the wild-type，it should be detected by high sensitive methods，such as PNA-PCR method. And the direct sequencing method should be used for mutant-type.