The purpose of this study was to understand the antimicrobial effects of cecropin-XJ expressed by genetic engineering from Xinjiang Silkworm Bombyx mori.To further characterize the antimicrobial activity of cecropin-XJ,antimicrobial curve was drawn.Ultrastructural observation of Staphylococcus aureus treated with cecropin-XJ was also performed.It was showed that the ability of cecropin-XJ to kill S.aureus was effective and time-dependent.It might be through the "barrel-stave pore" mechanism that cecropin-XJ quickly penetrated and damaged the cell membrane.And then cecropin-XJ affected cytoplasmic membrane,disturbed the ordered arrangement of membrane lipid,and changed permeability of cytoplasmic membrane,leading to the leaking out of cytoplasmic contents.Finally,the affected S.aureus died due to leakage of the cell contents.The cell wall kept intact after cecropin-XJ treatment,suggesting that cecropin-XJ does not damage cell wall.These results indicate that cytoplasmic membrane is the main target attacked by cecropin-XJ.