To investigate the effect of demethylating agent 5-Aza-CdR (5-aza-2'- deoxycytidine) on demethylation and transcription-regulating of RASSF1A gene in gastric cancer cell SGC7901 in vitro, as well as on the growth inhibition of cells. After SGC7901 cells were treated with 5-Aza-CdR, MTT assay, flow cytometry, and Annexin V-FITC staining were performed to analyze the cell proliferation, cell cycle and apoptotic rate respectively. Methylation- specific PCP (MSP), RT-PCR and Western blotting were used to detect methylation state, expression of mRNA and protein of RASSF1A gene. After SGC7901 cells were treated with different concentrations of 5-Aza-CdR, the cell growth was inhibited(P<0.05), the cell cycle was blocked at G(1) phase, and the apoptotic rate increased significantly(P<0.05). Hypermethylation was detected in the promoter region of RASSF1A gene in SGC7901 cells, and no expression of RASSF1A mRNA and protein was found. After treated with 5-Aza-CdR, demethylation occurred in RASSF1A gene,which subsequentl