OH-CATH is a novel cathelicidin identified from king cobra. It showed strong antibacterial activity against various bacteria in the presence of 1% NaCl and no haemolytic activity toward human red blood cells even at a high concentration. OH-CATH might serve as model molecules for the development of antimicrobial drugs. Understanding the action mechanism of OH-CATH and the reason for its selectivity against microbes is very important for this purpose. The bactericidal effect of the king cobra antimicrobial peptide OH-CATH on Gram-negative Escherichia coli (ATCC 25922) is observed by scanning electron microscopy (SEM) and transmitted electron microscopy (TEM). The SEM and TEM results suggested that the bactericidal mechanism of OH-CATH against Escherichia coli happened in three steps. Firstly, OH-CATH attached to the negatively charged bacterial wall by positively charged amino acid residues. In the second step, the accumulated OH-CATH aggregated and damaged the bacteria membrane in a pore-forming manner. In the last step, with the damage of cell permeability, the contents of the cells were released and eventually cells died.