Chlamydophila psittaci (Cps) is an obligate intracellular bacterium and can infect a broad range of mammalian species and cause variety clinical signs. Cps has a highly infectious ability to birds, poultry and livestock. Human infection can occur whenever there is close and continued contact with the infected birds.Major outer membrane protein (MOMP) represents the majority of the surface-exposed protein of membrane of Cps. It is a protein of around 40 kDa and is characterized by four variable sequences and five intervening constant regions of conserved structure and function.In the present paper, the Cps MOMP gene was amplified by PCR and inserted into pcDNA3.1( ) vector. The recombinant vectors were identified by PCR and enzyme digestion and results showed that the interest gene was successfully cloned to the pcDNA3.1( ) vector. The recombinant vector was also sequenced and result showed that the interest gene was inserted to the vector in the correct open reading frame. There was also a Kozak consensus sequence at the beginning of the insert MOMP gene.The recombinant plasmid was transformed into COS-7 cells by CaPO_4-mediated method. Western Blot was used to identify the expression of the protein and result showed that the interested gene was expressed successfully in COS-7 cells. A group of Balb/c mice was injected with 100μg recombinant plasmid and immunological response was observed. Indirect ELISA was used to test the antibody titer of the serum. Results showed that, after three times of injection, all of the mice showed specific antibody response. The immunological response persisted for a long time.