Accumulating evidence has demonstrated that arsenic trioxide (ATO) exhibits its anti-cancer activities in a variety of human malignancies. Recent studies have revealed that ATO regulated multiple microRNAs (miRNAs) in human cancers. However, the exact mechanism of ATO-mediated tumor suppressive function has not been fully elucidated. In the present study, we explore whether ATO governed oncogenic miR-27a in breast cancer cells by multiple methods such as MTT assay, RT-PCR, Wound healing assay, Western blotting analysis, migration, Transwell invasion assay, and transfection. Our results showed that ATO inhibited cell growth, migration, invasion, and induced cell apoptosis in breast cancer cells. Further molecular analysis dissected that ATO inhibited miR-27a expression in breast cancer cells. Moreover, inhibition of miR-27a suppressed cell growth, migration, invasion, and trigged cell apoptosis, whereas overexpression of miR-27a enhanced cell growth, motility, and inhibited apoptosis in breast cancer cells. Notably, we found that miR-27a inhibitor treatment potentiates ATO-induced breast cancer cell growth inhibition, apoptosis and motility inhibition. However, overexpression of miR-27a partly abrogated ATO-mediated anti-tumor activity. Our findings provide a novel anti-tumor mechanism of ATO involved in miR-27a for the treatment of breast cancer. (C) 2015 Elsevier Inc. All rights reserved.