In order to assess chicken T cell-mediated responses after immune stress, 200 two-week-old chickens were randomly divided into control group(C) and treatment groups (T₁ and T₂). The live I-type of Newcastle disease vaccine (ND) was taken as the source of immunological stress. The chickens in group (T₂) were injected with overdose of live I-type Newcastle disease vaccine. After vaccination, the dynamic changes of CD4⁺, CD8⁺T cells on thymus were detected by immunohisto-chemistry. The ultra-structure of T cells on thymus of the chickens in group (T₂) was observed by electron microscopic. The result showed: (1) After immune stress, from day 1 to day 5, the number of the CD4⁺T cells significantly declined and reached the bottom at day 5, but the number of the CD8⁺T cells increased dramatically and peaked at day 5. (2) After immune stress, from day 1 to day 5, part of T cells of chicken thymus came with apoptotic and pathological changes of putrescence continually. These results underscore: the immune stress can cause transient immune adjustment. These changes for chickens were the self-protection mechanism of immune system to adapt to survival and avoid immune disorder.