Abstract: Mannheimiosis is an important respiratory disease of cattle,sheep and other ruminants,and cause cattle and sheep industry great economic loss,Mannheimia haemolytica is the pathogen of this disease,this paper will give a brief overview of its biological characteristics,clinical symptoms,virulence factors,pathogenic mechanism and so on, and lay a foundation for further study.

Key words: Mannheimia haemolytica; pathogenic mechanism; free radical