Objective: Breast Cancer is the most common cancer in Iranian women.Breast tumors are classified based on the ESTROGEN RECEPTOR ALPHA (ERa) expression status into ER negative and ER positive tumors. ER negative tumors tend to have worse prognosis and less likely to respond to endocrine therapy.Aberrant methylation of gene promoter is one of the mechanisms for gene silencing in breast tumors. Because of its reversible nature, promoter methylation is a good target for new therapeutic strategies. We aimed to evaluate the frequency of this epigenetic event in ERa gene and its association to clinicopathological features in Iranian breast cancer patients. Materials and Methods: In this case control study the patient series consisted of 100 sporadic primary breast cancer cases (51 ER negative and 49 ER positive tumors). None of the participants had chemo or radiotherapy before surgery. In breast tumors ERα promoter methylation were assessed with methylation specific polymerase chain reaction (MSP). Data was collected on clinicopathological features of the patients. Correlation between ERα methylation and clinicopathological characteristics of the patients was investigated by Pearson Chi-Square and Fisher’s exact test.Results: ERa methylation was detected in 98% of ER negative and 65% of ER positive breast tumors. A strong correlation was found between ERa methylation and ER negativity in tumors (p<0.0001).Also, ERa methylation has associated to progesterone RECEPTOR negativity (p<0.008) and double RECEPTOR negative status (p<0.0001) in breast tumors.Conclusion: ERa methylation occurs with high frequency in the breast tumors of Iranian breast cancer patients and may play a considerable role in pathogenesis of ERa negative tumors as a poor prognosis and more aggressive category. The reversible nature of DNA methylation may provide new therapeutic possibilities in ER negative breast tumors.