In addition to its property of enhancing major histocompatibility complex (MHC) class II expression, the class II transactivator (CIITA) was recently demonstrated to be involved in T helper type 1/type 2 (Th1/Th2) differentiation by regulating interleukin‐4 (IL‐4) gene transcription. There was however, controversy regarding whether CIITA promotes or suppresses IL‐4 expression in the experiments with transgenic mice. To clarify the discrepancy by using simpler experimental systems, human Jurkat T cells that express IL‐4 but not interferon‐γ, even if stimulated with phorbol 12‐myristate 13‐acetate plus ionomycin, were used for CIITA transfection. Significant suppression of IL‐4 gene expression was demonstrated. Simultaneously, histones H3 and H4 in the IL‐4 promoter were hypoacetylated. The suppression could be totally reversed by the histone deacetylatase inhibitor trichostatin A. Furthermore, the IL‐4 expression was determined in primarily established human Th1/Th2 cells to which CIITA small interference RNA (siRNA) had been introduced. A substantially increased level of IL‐4 was recorded in the CIITA siRNA‐transfected Th1 cells, which was in parallel with significantly enhanced acetylation in histone H3 of the IL‐4 promoter. Chromatin immunoprecipitation analysis indicated that CIITA abrogated the binding of coactivator CBP/p300 and transcription factors STAT6/NFAT1 to IL‐4 promoter in the CIITA‐transfected cells. In conclusion, CIITA was active in the repression of transcription activation of human IL‐4 gene in both the T‐cell line and the primary human CD4 T cells by preventing transcription factors from binding to IL‐4 promoter through histone hypoacetylation. Our data confirm a potential significant role of CIITA in controlling Th1/Th2 differentiation via modulation of IL‐4 gene activation.