Abstract: The experiment was conducted with the objective of examining the effects of high molybdenum on the thymus in the broilers by the methods of experimental pathology and flow cytometry(FCM)．360 one-day-old Avian broilers were randomly divided into four groups，and fed on diets as follows：Control diet (Mo 13 mg·kg^-1) and High molybdenum diets(Mo 500 mg·kg-1, High molybdenum groupⅠ; Mo 1 000 mg·kg-1, High molybdenum groupⅡ; Mo 1 500 mg·kg^-1, High molybdenum group Ⅲ) for 6 weeks. The results showed that decreased lymphocytes, enlarged Hassall′s corpuscles and increased myoid cells were observed in high molybdenum groupsⅠ,Ⅱand Ⅲ in comparison with those of control group histopathologically. The weight and the relative weight of thymus were decreased. The statistical analyses by FCM indicated that the G0/G1 phase of thymocyte was increased (P<0.05 or P<0.01) and the G2+M phase, S phase of thymocyte and the PI (Proliferating index) were decreased (P<0.05 or P<0.01) in high molybdenum groupsⅠ,Ⅱ and Ⅲ. Meanwhile, the percentage of cellular apoptosis was higher in high molybdenum groupsⅠ,Ⅱ and Ⅲ than that in control group. Also, the TUNEL staining was consistent with the result of FCM. It was concluded that dietary molybdenum in excess of 500 mg·kg^-1 inhibited the development of thymus, caused pathological changes in the thymus and impaired the function of cellular immunity in broilers.