Abstract: SPF birds were inoculated with avian leukosis virus subgroup J- NX0101 strain at 1 day age and 7 day age, respectively-analog congenital infection and early infections. Detect the effect of immune system caused by ALV-J. Body weight, immune organs weight, histopathology, blood cells, CD4⁺ and CD8⁺T lymphocytes in spleen and thymus were tested. Results showed that ALV-J suppressed significantly not only body weight but also immune organs, especially in thymus and the bursa of Fabricious from 2 weeks age. The most serious immunosuppression occurred post inoculation 3 to 4 weeks. Histopathology observation found that most immune cells were destroyed and interstitial connective tissue proliferation in thymus and the bursa of Fabrisious. The other organs showed inflammatory infiltrate and hemorrhage. Tumor was not seen in any organs. Blood detection showed that ALV-J caused granular leukocytes, lymphocytes and erythrocytes decreasing, especially group 1. CD4⁺ and CD8⁺T lymphocytes detection in spleen and thymus showed that CD4⁺ cell number decreased, while CD8⁺ cell number increased at 5 weeks age. It is confirmed that the immunosuppression caused by ALV-J was high related with the two kinds of cells. The study demonstrated that the latent period of ALV-J in vivo was at 3 to 4 weeks after infection.