Oligochitosan has shown a great potential as an environmental safe agent to control plant disease due to its unique dual function: antifungal effects and elicitor activity. Recently, mechanisms by which oligochitosan elicits the defense response of plant have been intensively studied. However, little attempt has been made to elucidate the mechanisms of antifungal activity of oligochitosan. In this work, antifungal activity and mode of action of oligochitosan against phytopathogen were investigated in vitro and major findings were listed as follows:1. The antifungal activity of oligochitosan against different phytopathogens in vitro was tested by mycelial radial growth method and Phytophthora turned out to be most sensitive one (EC50≤100mg L-1). Oligochitosan was thermal stable and its inhibition of P. capsici showed little variance over the range of temperature for mycelium growth. Antifungal activity of oligochitosan was greatly reduced when pH values was adjusted to 7.2. Using CLSM, internalization of oligochitosan in P. capsici was observed for the first time in fungi. Uptake of oligochitosan by P. capsici was time- and concentration- dependent and energy-driven. Monosaccharides such as glucosamine and mannose had no effect on the internalization of oligochitosan.3. Inhibition ofβ-1,3-glucanase from P. capsici by oligochitosan was found for the first time, and addition of commercialβ-1,3-glucanase reduced the inhibition of oligochitosan on P. capsici.4、Increase of cell membrane electrolytes permeability, measured by change of electrical conductivity of medium of P. capsici, was found upon treatment with oligochitosan. Cell membrane rupture of zoospore in the presence of oligochitosan was observed and this effect was reduced by addition of glucose (osmotic regulator) but not ATP(energy supplier). The most profound ultrastructural alteration of P. capsici treated with oligochitosan was the disruption of the endomembrane system, especially vacuole and secretory vesicles such as plasmalemmasomes.5、The result of gel-retardation experiment provided the first evidence for direct interaction between oligochitosan and P. capsici nucleic acids. Fractionations of soluble protein of P. capsici were achieved using magnetic cross-linked chitosan and proteins in one fraction showed hemaggluting activity.