This study aims at evaluating the prognostic significance of neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), lymphocyte-to-monocyte ratio (LMR), and systemic immune-inflammation indexes (SII) in metastatic colorectal cancer (mCRC) patients treated with cetuximab. Ninety-five patients receiving cetuximab for mCRC were categorized into the high or low NLR, PLR, LMR, and SII groups based on their median index values. Univariate and multivariate survival analysis were performed to identify the indexes' correlation with progression-free survival (PFS) and overall survival (OS). In the univariate analysis, ECOG performance status, neutrphil counts, lymphocyte counts, monocyte counts, NLR, PLR, and LDH were associated with survival. Multivariate analysis showed that ECOG performance status of 0 (hazard ratio [HR] 3.608, p < 0.001; HR 5.030, p < 0.001, respectively), high absolute neutrophil counts (HR 2.837, p < 0.001; HR 1.922, p = 0.026, respectively), low lymphocyte counts (HR 0.352, p < 0.001; HR 0.440, p = 0.001, respectively), elevated NLR (HR 3.837, p < 0.001; HR 2.467, p = 0.006) were independent predictors of shorter PFS and OS. In conclusion, pre-treatment inflammatory indexes, especially NLR were potential biomarkers to predict the survival of mCRC patients with cetuximab therapy.