Selenium is an essential trace element possessing immune-stimulatory properties. The purpose of this 42-day study was to investigate the effects of excess dietary selenium on cellular immune function by determining morphological changes of thymus and peripheral blood T-cell subset. Three hundred 1-day-old avian broilers were fed on a basic diet (0.2 mg/kg selenium) or the same diet amended to contain 1, 5, 10, 15 mg/kg selenium supplied as sodium selenite (n = 60/group). Pathological lesions were progressed with the dietary Se level increased. Grossly, the volume of thymus was decreased. Histopathologically, lymphopenia and congestion were observed. Ultrastructurally, mitochondria injury was observed. In comparison with that of control group, 5, 10 and 15 mg/kg dietary Se decreased the percentage of CD3(+), CD3(+)CD4(+) and CD3(+)CD8(+) T cells of the peripheral blood, as measured by flow cytometry. The results showed that excess selenium (more than 5 mg/kg) intake could cause lesions of thymus and decrease of T-cell subsets. The cellular immune function was finally impaired in broilers.