The species of active oxygen(AO) and its metabolism, anti-oxidative mechanism in plant cell, assay for AO, and the production of AO in interaction between pathogen and plant and so on were reviewed in this paper. Based on these, it was speculated that some key processes such as signal transduction, antimicrobial effect, membrane lipoxidation, cell wall modification, phytoalexin synthesis and hypersensitive cell death result in from metabolism of active oxygen.