Defensins are small (3-5kD) cysteine-rich cationic proteins found in both vertebrate and invertebrates. They are important components of innate immunity against microorganisms including bacteria, fungi and enveloped viruses. Human β-defensin (hBD3) C-terminal peptide analog (Y2) has been proven with higher antibiotic effect against bacteria Pseudomonas, while it shows lower cytotoxic effect on mammalian cells comparing to natural hBD3 protein. The other analog C2 is much less effective. In our study, Y2 and C2 structures in aqueous solution, micelles (DPC), and bicelles (DMPC and DHPC) are analyzed and compared. NMR structures show that Y2 peptide undergoes dramatic conformational change upon interacting with lipid membranes. This peptide may penetrate prokaryotic cell membranes by forming a pore in the membrane which allows efflux. Y2 structure analysis and bacterial killing mechanism study will be useful for antibiotic drug design. [5th Awards]