Acetylation of histone is a major player in epigenetic modifications, resulting in open chromatin structures and, hence, permissive conditions for transcription-factor recruitment to the promoters, followed by initiation of transcription. Histone deacetylase inhibitors arrest cancer cell growth and cause apoptosis with low toxicity thereby constituting a promising treatment for cancer. In this study, we examined the antiproliferative effects of valproate with a combination of nicotinamide in the MCF-7 cell line. MCF-7 was treated with various concentrations of valproate. The MTT assay showed that the viability of MCF-7 cells was inhibited and the cell activity was decreased. Viability percent of valproate and nicotinamide combined treatment cells (28 ± 2) was 1.78 times increased compared with the valproate-alone (0.5 mM) treated cells (50 ± 2). Colony formation in soft agar indicated that valproate at 0.3 mM, when used alone, weakly suppressed proliferation of cells (82 ± 3) and the combination treatment of valproate + nicotinamide strongly suppressed cell proliferation (51 ± 3). The flow cytometric and microscopic analyses of HDACI combined with treated cells indicated strong apoptosis induction and nuclear morphological alterations greater than those of valproate alone. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) analysis confirmed the efficiency of the HDAC inhibitor combination, revealing the effectively upregulated p16 and p21. Furthermore, to investigate the role of acetyl-histone H3 levels, western blot analyses have been performed and high levels of acetylated histone H3 were detected in valproate- and nicotinamide-treated cells. These results suggest that the combination treatment of valproate with nicotinamide exerts significant antitumor activity and could be a promising therapeutic candidate to treat human breast cancer.