The purpose of this study was to investigate the effects of methionine deficiency on cellular immune function by determining morphological and ultrastructural changes of thymus, thymic cell cycle and apoptosis, peripheral blood T-cell subsets, Tcell proliferation function and the serum interleukin-2 (IL-2) contents. 120 1-d-old broilers were randomly divided into two groups (6 replicates in each group and 10 broilers in each replicate) and fed on a control diet or methionine deficient diet for 42 d. Lesions were observed in experiment. Histopathologically, lymphopenia and congestion were observed in the medulla of thymic lobule. Ultrastructurally, there were more apoptosis lymphocytes, and the mitochondria of lymphocytes were swelled in thymus of methionine deficiency. The G0/G1 phase of the cell cycle of the thymus was much higher (P<0.01), and the S, G2+M phases and proliferating index (PI) were lower (P<0.01) in methionine deficiency than in control group. And the percentage of apoptotic cells in the thymus was significantly increased in methionine deficiency (P<0.01). The percentage of CD4+ and CD8+ T-cells was decreased in methionine deficiency compared with control group. Meanwhile, the proliferation function of peripheral blood T-cell was decreased in methionine deficiency. Also, the serum IL-2 contents were decreased in methionine deficiency. It was concluded that methionine deficiency could cause pathological and ultrastructural changes of thymus, reduce the T-cell population, serum IL-2 contents and the proliferation function of Tcells, and induce increased percentage of apoptotic cells. The cellular immune function was finally impaired in broilers.