Our purpose was to investigate the effect of 3-deazaneplanocin A (DZNep) on human T-cell acute lymphoid leukemia (T-ALL) cells, and to explore the underlying molecular mechanisms. The human T-ALL cell line Molt4 was treated with DZNep, and cell proliferation was examined. The expression of Mps one binder kinase activator 1 gene (MOB1) mRNA and protein was determined by RT-PCR and Western blotting, respectively. The histone modification effect of DZNep on the lysine 9 of histone 3 associated with MOB1 promoters was examined with chromatin immunoprecipitation and quantitative PCR, and CpG methylation in MOB1 promoters was detected by bisulfite sequencing PCR. DZNep treatment inhibited the growth of Molt4 cells. The expressions of MOB1 genes were upregulated by DZNep treatment, and histone methylations in their promoters were significantly reduced. The results indicate that DZNep is a promising therapeutic compound for the treatment of human T-ALL.