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Experimental study was conducted to examine the effect of copper excess on the peripheral blood Tlymphocyte by the methods of flow cytometry (FCM) and experimental pathology.420 one-day-old Avian chickens were randomly divided into seven groups, and fed on diets as follows: 1 .controls (Cu 11mg/kg)and 2.copper excess( Cu 100mg/kg, copper excess group Ⅰ; Cu 200mg/kg, copper excess group Ⅱ; Cu 300mg/kg, copper excess group Ⅲ; Cu 400mg/kg, copper excess group Ⅳ; Cu 500mg/kg, copper excess group V;Cu 600mg/kg,copper excess group Ⅵ) for six weeks.The results were as follows: 1) In thymus, lymphocytes in the medulla were decreased in number in copper excess groups Ⅲ, Ⅳ,Ⅴ and Ⅵ,and the increased and enlarged thymic corpuscles and the proliferated reticular cells were also observed in both copper excess group Ⅴ and copper excess group Ⅵ in comparison with those of control group.2) The percentage of CD4 + T cells was markedly decreased from 2 to 6 weeks of age in copper excess groups Ⅳ,Ⅴ and Ⅵ (P＜0.05 or P＜0.01).3) The percentage of CD8+ T cell was not varied in six copper excess groups during the experiment when compared with that of control group ( P＞0.05).4) The CD4+ /CDs + ratio was lower from 2 to 6 weeks of age in copper excess groups Ⅳ, Ⅴ and Ⅵ than in control group (P＜0.05 or P＜0.01).5) It was concluded that dietary copper in excess of 300rag / kg suppressed the development of T-lymphocytes and reduced the percentage of CD4+ T ceils and the CD4+/CD8+ ratio, and resulted in pathological injury of the thymus.Cellular immune function was finally impaired.