Hermetia iIlucens fat includes balanced and unique medium-chain fatty acids (MCFAs), which act as a prophylactic agent against many microorganisms. MCFAs are one of the pioneering ways to act as antimicrobial fatty acids. Escherichia coli (E. coli) is one of the Enterobacteriaceae which threaten broiler production. H. illucens oil (HIO) is enriched with MCFA, mainly lauric acid (LA), which has antibacterial activity. Florfenicol (FFC) is a broadspectrum antibiotic that works against Gram-negative bacteria. Excessive use of antibiotics induces bacterial resistance, which is a serious problem facing animal health. In the present study, we aim to evaluate the influences of H. illucens MCFAs as a prophylactic agent versus chemical treatment by FFC on the growth performance and histomorphological character of the intestine in broilers infected by E. coli O157 strains. One-hundred-day-old chicks were divided into 5 groups, 20 chicks for each. Group 1 was assigned as control negative and administered only saline. Group 2 was a control positive (non-treated) and infected with E. coli serotype O157. Group 3 was infected by E. coli and administered H. illucens fat. Group 4 was infected and FFC-treated. Group 5 was infected and treated with both FFC + H. illucens fat. The results showed that MCFAs in H. illucens fat promote the growth rate of broilers, provide significant body weight gain, and improve their intestinal mucosal structure compared to the control positive group. In conclusion, H. illucens MCFA can be used as a natural growth promoter instead of the chemical antibiotic FFC.