Chronic myeloid leukemia (CML) is associated with overexpression of BCR-ABL1, a nonreceptor tyrosine kinase critical for malignant transformation. We investigated whether non-coding microRNAs (miRNAs) targeting BCR-ABL1 mRNA contribute to the pathogenesis of CML. Indeed, miR-30a targeted BCR-ABL1 and was underexpressed in bone marrow from CML patients. In K562 leukemia cells, overexpression of miR-30a reduced ABL1 and BCR-ABL1 protein expression, decreased proliferation, and arrested cell cycle progression between G1 and S. These findings strongly suggest that miR-30a acts as a tumor suppressor by downregulating ABL1 and BCR-ABL1 expression. Upregulation of miR-30a in hematopoietic cells may have therapeutic efficacy against CML.