Histone deacetylases (HDACs) are overexpressed in various types of primary human cancer and have become attractive targets for cancer therapy. We designed and synthesized a series of new class of HDAC inhibitors (HDACi). Among these, S-(E)-3-(1-(1-(benzo[d]oxazol-2-yl)-2-methylpropyl)-1H-1,2,3-triazol-4-yl)-N-hydroxyacrylamide (NK-HDAC-1) showed potent antitumor activity. In the present study, we examined the antitumor effects of NK-HDAC-1 on breast cancer in vitro and in vivo. The inhibitory effects of NK-HDAC-1 on HDAC enzyme activity and cell growth were more potent compared to suberoylanilide hydroxamic acid (SAHA). NK-HDAC-1 caused G1 cell cycle arrest at concentrations below 0.2 µM and G2/M arrest at concentrations above 0.4 µM through p21 upregulation and cyclin D1 downregulation. NK-HADC-1 induced hyperacetylation of histone H3 and H4 around the promoter region of p21. NK-HDAC-1 promoted apoptosis in MDA-MB-231 breast cancer cells by activating both the intrinsic and the extrinsic pathway NK-HDAC-1 at doses of 3, 10 and 30 mg/kg reduced the tumor volume in MDA-MB-231 xenografts by 25.9, 48.8 and 63.6%, respectively. The results suggested that NK-HDAC-1 may be a promising therapeutic candidate in treating human breast cancer.