Abstract: Antigenicity and immunogenicity of 25 H9N2 subtype avian influenza viruses isolated from China during 19982008 were compared and analyzed by crossinghemagglutinin inhibition test(HI), cells neutralization test and challenging test. HI test results revealed that the 25 H9N2 AIVs were divided into three classes, the first class including 12#, 17#, 18#, whose HI titer was from 4.0 log2 to 5.0 log2, the second including 2#, 5#, 11#, 15#, 16#, 21#, 22# and 25#, whose titer was from 6.0 log2 to 7.0 log2, the rest strains formed the third whose titer was from 7.0 log2 to 8.0 log2. It was indicated that the antigenicity of different isolation strains of H9N2 AIV displayed variant, and Maximum difference of them in HI titer reached to 3.15 log2. Neutralization test demonstrated that the antigen correlation(R) among 8 representative H9N2 AIV strains selected were from 0.42 to 0.84, the antigenicity between 3# and 14#, 11# and 17# showed diversity (the R was 0.46 and 0.42), but that of all the rest strains showed inconspicuous difference. Cross challenging protection test among 6 representative H9N2 AIV strains selected made clear that the antibody induced by the vaccine from the viruses could protect the chicken against infection. The birds vaccinated with Hp vaccine could be protected against the challenge from both three H9N2 AIV strains (10#, 12#, 17#) displaying different antigenicity, and jointchallenge with different viruses (9# and15#,16# and 17#). It was suggested that the radical change in immunogenicity of the prevalent H9N2 AIVs did not happen up to now.