Antimicrobial peptides of the cathelicidin family are found in many mammalian species, and are focused on various effects other than antimicrobial action. In this study, we evaluated the anti-proliferative effect of an analogue peptide, FF/CAP18, derived from an endogenous cathelicidin family member against the colon cancer cell line HCT116. FF/CAP18 significantly decreased the proliferation of HCT116 cells in a dose-dependent fashion. Furthermore, the treatment of HCT116 with FF/CAP18 caused loss of mitochondrial membrane potential, and resulted in the immunoreactivity to the single-strand DNA antibody, suggesting the early stage of apoptosis. Interestingly, the anti-proliferative effect of FF/CAP18 was constant regardless of the genotype of p53 (wild-type and p53 mutant type HCT116 cells). Therefore, the signaling pathway of p53 is not involved in the growth suppression effect of the cathelicidin analogue peptide. These results indicate that the treatment of certain types of cancer cells with FF/CAP18 may increase the sensitivity of the chemotherapeutic reagents, which might relate to the reduction of the side effects.