Objective        To determine the inhibitory effects of arsenic trioxide (As

2

O

3

) on the proliferation in hormonal independent prostate cancer DU145 cells

and on demethylation and expression of Ras-assciation domain family 1A (RASSF1A).       Methods        MTT assay was used to test the growth of DU145 cells after the treatment of As

2

O

3

at 0.5

1.0

2.0

4.0

6.0

12.0 or 20.0 μmol/L for 24

48 or 72 h. Methylation-specific PCR (MSP) a

nd Western blot analysis were used to detect the methylation status and protein expression of RASSF1A gene in Du145 cells after treatment.       Results        As

2

O

3

significantly inhibited the proliferation of DU145 cells in a dose- and time-dependent manner (F=838.089

P0.05

F=8.849

P0.05). As

2

O

3

treatment reversed the methylation status of RASSF1A gene and made the protein re-expression.       Conclusion        As

2

O

3

reverses abnormal methylation status of promoter CpG island of RASSF1A gene in DU145 prostate cancer cells

induces protein re-expression and inhibits the cells proliferation.