Chitosan oligosaccharides COS-N conjugate were prepared by Maillard reaction in solution condition. Infrared spectroscopic analysis and scanning electron microscope showed that COS and NISIN are cross-linked by covalent bonds， and the structure were also changed. The median lethal dose（LD50） of COS-N Mixture and conjugate on Escherichia coli and Staphylococcus aureus were measured. The results showed thatunder the same testing conditions， the LD50 of COS-N Conjugate to Escherichia coli and Staphylococcus aureus was reduced by 22% and 55% respectively. when compared with that of COS-NMixture. The study on the antimicrobial mechanism showed that COS-N conjugate could destroyed the integrity of cell membrane， released the intracellular substances and increased the permeability of bacterial cell membrane to a greater extent， when compared with that of COS-N Mixture. In summary， Maillard reaction can effectively improve the antibacterial activity of COS-N Conjugate， which can a novel way to synthesis natural preservatives.