Abstract: By the methods of FCM and MTT, the subsets and lymphocyte proliferation function of peripheral blood T lymphocyte were studied in AA poultries by feeding with sufficient (0.2 mg·kg^-1, control group), or excess selenium (1, 5, 10, and 15 mg·kg^-1), as sodium selenite in the diets to five groups of 60 chickens each for 6 weeks. The results showed that the percentage of CD3⁺ T cells, CD3⁺CD4⁺ T cells and CD3⁺CD8⁺ T cells were decreased variably in chickens when dietary selenium exceeded 5 mg·kg^-1. The mitogenic response of T lymphocytes to ConA and the content of serum IL-2 were decreased in high Se group Ⅲ and high Se group Ⅳ (P<0.05 or P<0.01) when compared with control group. The results suggested that the dietary selenium in excess of 5 mg·kg^-1 impaired the cellular immune function in chickens.