The infection cushion of penetration the leaf of oilseed rape was investigated by transmission electron microscopy and scanning electron microscopy. During the initial infection process, the hypha of grey mould fungus （Botrytis cinerea ） which formed infection cushions broken down the cell wall then penetrated into the leaf tissue of oilseed rape. After invading into the leaf epidermis cells, the hypha of infection cushions multiplied between the mesophyll cells or grew in mesophyll cells through breaking the cell wall. These results will illustrate the degradation of plant cell wall plays an important role in the infection and development of grey mould fungus infection cushion and provide clues to explain pathogenicity of grey mould fungus.