Active oxygen species (AOS) play important roles in the interaction between host and pathogen. Though it was long considered that AOS released in the interaction between host and pathogen was primarily mediated by a membrane-bound redox complex in host, further, present studies showed that AOS, which had potential relation to pathogenicity of pathogen, was also detected in pathogen and took part in the interaction. AOS production was regulated by complicated mechanisms in pathogen. These findings provide potentially important clues in discovering the mechanism of host-pathogen interaction. In general, this review describes the ubiquitous phenomenon of AOS generation in many pathogens, including bacteria and fungi. The AOS regulatory mechanism, such as releasing sites and functional molecules, was demonstrated. AOS generation in plant pathogen is significant even though further studies are needed. The discussion was also made on the phenomenon, likely regulatory mechanism and the pathological significance of AOS generation in plant pathogens.