Abstract: Objective :To study the inhibitory effects of HER2 specific antisense oligodeoxynu-cleotide HA6722 administered alone or in combination with β -elemene on proliferation of breast cancercell lines. Methods :MDA-MB453 and MDA-MB231 cell lines, which are HER2 over-and low-ex-pressors, respectively, were used as experimental cells. Inhibitory effects of HA6722 alone or in combi-nation with β-elemene on these cells were detected by means of methyl thiazolyl blue (MTT). Results :HA6722 and β-elemene could both inhibit the growth of MDA-MB453 cells in vitro in a dose-depen-dent manner with IC₅₀ values of 41.8±8.1nmol · L^-1 and 79.4±12.1nmol · L-1 (n=3), respectively. Whenadministered in combination with a low dose of HA6722, inhibitory effects of β - elemene on MDA-MB453 cells increased markedly, resulting in significant decrease of IC 50 from 79.4±12.1 nmol · L-1 to51.6±8.2 nmol · L^-1 (n=3, P<0.05). The combination index (CI) was 0.72±0.38 (n=3), which indicated asynergistic effect of these two drugs on HER2- overexpressing breast cancer cells. Conclusion : Whenadministered in combination, antisense oligodeoxynucleotide HA6722 and β - elemene inhibit the growthof HER2- overexpressing breast cancer cells in a synergistic manner.