The effects of 5-Aza-2'-deoxycytidine (5-Aza-CdR) on the proliferation of MDA-MB-435S cells and the expression of tumor suppressor gene maspin were investigated. Human breast cancer cell line MDA-MB-435S was treated with 5 μmol/L 5-Aza-CdR, a specific demethylat-ing agent for 0 to 8 days. The growth of MDA-MB-435S cells was observed by MTT assay before and after 5-Aza-CdR treatment, respectively. The expression of maspin mRNA was detected by re-verse transcription-polymerase chain reaction (RT-PCR). The cell cycle of MDA-MB-435S cells was analyzed by flow cytometry. The results showed that the growth of MDA-MB-435S cells treated with 5-Aza-CdR for 8 days was significantly suppressed as compared with the control groups, and the in- hibition rate increased sharply from 5 day to 8 day (35.42% to 71.29%). Flow cytometry showed that 5 μmol/L 5-Aza-CdR could induce G2/M cell cycle arrest and decrease the percentage of mitosis cell number in this cell line. Maspin mRNA was expressed in MDA-MB-435S cells after 5-Aza-CdR treatment, but it was weakly detectable before the treatment. It was concluded that Maspin gene might be transcriptional silencing by hypermethylation and the re-expression of maspin gene by 5-Aza-CdR can inhibit the proliferation and induce the G2/M arrest of MDA-MB-435S breast cancer cells.