Antifungal activities of chitosans (CTS) with different molecular weights (MW)and different concentrations against Fulvia fulva (cooke) ciffrri (F.fulva) causing leaf mould in tomato plants were studied in vitro and in vivo,the action mode and its inhibition at different stages during the life cycle of F.fulva were observed.Results showed that;(1)In vitro,CTS exhibited strong antifungal activity against F.fulva,especially for the medium MW (213 kDa and 499 kDa) CTS.Almost complete inhibition ofF.fulva conidia germination and mycelia colony radial growth was found when CTS was at concentration of 0.5 and 2mg ml-1 respectively,however,inhibitory effect on sporulation was not very obvious for all CTSs tested in this experiment.In vivo,CTS of 213 kDaCTS at 6mg ml-1 concentration produced stronger antifungal effect than others.(2) The morphological study by scanning electron microscope (SEM) showed that CTS could induce the hypha swelling,and the surface of hypha which was treated with low MW (82 kDa)chitosan was smooth but was rough treated with high MW (1320 kDa) chitosan.The further study using a confocal laser scanning microscopy (CLSM) coupled with fluorescein isothiocyanate (FITC)-fluorescence detection system showed fluorescence of the FITC-labeled chitosans of which MWs were below 500 kDacould enter into the inner of hypha,however,1320 kDa chitosan was blocked off the outer of hypha.