[Objective] To analyze the pathogenic mechanism of avian influenza A (H5N1) viruses in mice by investigating the changes of the immune system. [Methods] To investigate CD3+T cells, CD4+T cells, CD8+T cells by the flow cytometry. [Result] Avian influenza A(H5N1) virus destroyed the immune system, CD3+T cells, CD4+T cells, CD8+T cells declined, while the ratio of CD4+T cells/CD8+T increased, the tendency was same in blood as well as in spleen. [Conclusion] Avian influenza A(H5N1) virus can inhibit cell immunity; especially the CD8+T.