Based on experimental inoculation of chickens and sequence of amino acids at cleavage site, H9N2 AIV is pathotyped as low pathogenic avian influenza virus. But our extensive field experiences during last decade show serious disease problems and high mortality associated with this subtype in some Asian countries. One of the possible explanations for such a high mortality and great economic losses could be circulation of the virus and mixed infection with other respiratory pathogens. Infectious Bronchitis Live Vaccine (IBLV) is being used broadly in chicken farms of these countries. So it was decided to experimentally study the effect of infectious bronchitis live vaccine (H120) on enhancing of pathogenicity of H9N2 in broiler chicks. Clinical signs, gross lesions, viral shedding and mortality rate were compared between groups. Results of the present study showed that co-infection of IBLV with H9N2 AI virus not only increased the severity of H9N2 AIV clinical sings and gross lesions; but also increased the mortality rate and extended viral shedding period of H9N2 avian influenza virus.