Abstract: ObjectiveTo study the expression of Estrogen receptor-α(ERα) after treatment with different concentration of arsenic trioxide in breast cancer cells MDA-MB-231, and explore its mechanism. MethodsERα negative human breast cancer cells MDA-MB-231 were treated by arsenic trioxide, methylation-specific PCR (MSP) was used to detect the methylation status of CpG island of the promoter of ERα gene, reverse transcriptase PCR (RT-PCR) was used to detect the expression level of ERα mRNA. The ERα-positive human breast cancer cells MCF-7 was used as the positive control. ResultsThe CpG island of the promoter of ERα was hypermethylation in MDA-MB-231 cells, CpG island was demethylated and ERα mRNA expression was restored after treatment by As2O3. ConclusionCertain concentration of As2O3 can demethylate the CpG island of the promoter of ERα in breast cancer cells MDA-MB-231 and restores its mRNA expression.