Background: We have recently reported that Lactobacillus GG (LGG) elaborates seven small peptides that exert both anti- Gram-negative and Gram-positive bactericidal activity. Among them, peptides NPSRQERR, VHTAPK and PDENK have higher activity than others. Here we further report the inhibitory function of these three peptides on antibiotic- resistant bacterial strains and preliminary structure-activity analysis. Methods: Peptides were synthesized, purified, and dissolved in related media. Lactobacillus GG conditioned medium was used as a positive control and media alone or homologous-conditioned medium were used as negative controls. A600 measurement was used to measure antibacterial activity. Results: LGG-derived peptides NPSRQERR, VHTAPK and PDENK were found to inhibit growth of kanamycin-resistant E. coli SM10 δpir and tetracycline-resistant E. coli TOPO10. Moreover, growth of methicillin-resistant Staphylococcus aureus (MRSA) was also found to be inhibited by peptides NPSEQERR, VHTAPK and PDENK. Structurefunction analyses revealed that the anti-bacterial properties of these peptides can be increased or decreased by modification of amino acid charge.