The experiment was conducted to examine the effect of copper toxicity on lymphoid organs by experimental pathology and flow cytometry (FCM). 180 one-day-old Avian broilers were divided into three groups, and fed diets as follows: 1) Control(Cu 11.97 mg kg-1 diet), 2) Cu- toxic group Ⅰ (Cu 650 mg kg-1) and 3) Cu- toxic group Ⅱ (Cu 850 mg kg-1) for six weeks.Compared with the control, the growth index of the thymus, spleen and bursa of Fabricius were markedly reduced (P＜0.05or P＜0.01), the G0/G1 phase of cell cycles of the thymus, spleen and bursa of Fabricius was higher (P＜0.05 or P＜0.01), while the S phase and proliferating index were lower (P＜0.05 or P＜0.01)in both Cu-toxic group Ⅰ and Cu-toxic group Ⅱ. The results demonstrated that Cu toxicity seriously impaired the progression of lymphocytes from the G0/G1 phase to the S phase, inhibited the growth and development of lymphoid organs.