To confirm the inactivating effect of chito-oligosaccharides on Tobacco mosaic virus (TMV) par ticles in vitro, the difference of TMV pathogenicity was evaluated according to the decrease of local lesion numbers after inoculating with TMV mixed with chito-oligosaccharides (DP3-10) in Nicotiana glutinosa, and the virion structural change was studied by transmission electron microscopy after mixed with chito-oligosaccharides. In the range of tested concentrations of chito-oligosaccharides (100-1000 microg /mL), the numbers of local lesions were strongly reduced with over 30% decrement, and the 88.4% reduction gained at the concentration of 600g /mL. It revealed that treatment with chito-oligosaccharide solution of 300-500 microg /mL directly broke TMV particles into tiny pieces of 50-150nm long, and that treatment with solutions of 600-1000 microg/mL caused virus particle agglomerated. The data presented here suggested that chito-oligosaccharides exerted strong inactivating effect on plant virus in vitro.