Abstract: OBJECTIV E To investigate the synergistic effect of decitabine(DCA) and valproic acid(VPA) on non-metastasis 23-H1 (nm23-H1) gene expression in gastric MGC-803 cells. METHODS The groups were set as follows: DCA 1.5 and 3.0 μmol·L^-1, VPA 1.5 mmol·L^-1, DCA 1.5 μmol·L-1+VPA 1.5 mmol·L^-1, DCA 3.0 μmol·L^-1+VPA 1.5 mmo·L^-1. The cells were treated by drug for 72 h. The early and late apoptosis rates were detected by staining with Annexin V and PI. The nm23-H1 mRNA expressions levels were detected by real-time quantity PCR. Methylation status of 2 CpG island selected randomly was detected by pyrosequencing. RESULTS The apoptosis rates of VPA 1.5 +DCA 1.5 group early: (33.58±3.88)%, late: (31.52±4.20)% and VPA 1.5+DCA 3.0 early: (42.61±4.23)%, late: (38.01±3.86)% were significantly higher than their corresponding concentration single drug groups (P<0.01). The nm23-H1 mRNA relative expression in VPA 1.5+DCA 1.5 group (1.84±0.46) and VPA 1.5+DCA 3.0 group(3.02±0.36) were significantly higher than their corresponding concentration single drug groups(P<0.01). The methylation percentages of 2 CpG islands of nm23-H1 promoter in VPA 1.5 +DCA 1.5 group site1: (53.50±3.39)%, site2: (51.17±2.71)% and VPA 1.5+DCA 3.0 group site1: (41.17±2.14)%, site2: (39.83±2.56)% were significantly lower than their corresponding concentration single drug groups(P<0.01). The histone deacetylases(HDAC) activity of each of VPA 1.5mmol·L^-1, VPA 1.5+DCA 1.5, VPA 1.5+DCA 3.0 groups was significantly lower than either of Normal control, DCA 1.5 μmol·L^-1, DCA 3.0 μmol·L^-1 groups(P<0.01). CONCLUSION The induction of nm23-H1 mRNA expression by combinations of DCA and VPA is dependent on decreasing the methylation status of nm23-H1 promoter and reducing HDAC activity in gastric MGC-803 cells.