[Objective]Invading period and site of Ustilaginoidea virens( cooke) Tak under natural infection conditions were understood to provide reference for further study the invasion mechanism. [Method] In booting stage the floret organs of rice young panicle were observed by optical microscope to demonstrate the invading process of Ustilaginoidea virens( cooke) Tak. The resistance of rice male sterile lines was identified. [Result] The fungus invaded the floret through the crack between up and lower lemmas before...