Objective Chlamydia psittaci is now suspected as a primary predisposing factor eliciting a sever respiratory syndrome. Methods The broiler blood samples suffered from respiratory symptom were screened in Beijing and Tianjing areas during 2001 to 2002.The lung, heart and kidney tissue from infected broiler were fixed,stained,implanted and observed under microscope; meanwhile the lung tissue suspension from infected broiler was inoculated into the SPF egg on day 7. The yolk membrane of the dead chick embryo were collected during day 4 to 9,dyed by Lugols Iodine and by Chlamydia Immunofluorescence Assay. In order to discriminate Chlamydia psittaci from Chlamyida trachomatis,the activity of isolated pathogen was inhibited by infusing Penicilln and sodium sulfadiazine.The isolated pathogen was injected into broilers of 15 day,observering the clinical pathology. Results The serological response was seen after the clinical signs, and the positive rate varied from 10%-30%.The activity of pathogen was demonstrated positive in fluorochrome.The same pathological change could be replicated by injecting the isolated pathogen. Conclusion The isolated pathogen may be an Chlamydia psittaci, whose conbined infection with E.Coli can be responsible for the severe respiratory disease with high mortality in broilers.