Objective To explore the effect of microRNA-449a (miR-449a) on the biological behavior of breast cancer cells by targeting lysine demethylase 3A (KDM3A).Methods Real-time fluorescence quantitative PCR and Western Blot were used to detect the expression levels of miR-449a and KDM3A in breast cancer tissues and adjacent normal tissues, human normal breast epithelial cell line (MCF10A) and human breast cancer cell lines (MDA-MB-231, BT-474, MCF-7, SK-BR-3), and dual luciferase reporter gene assay was used to analyze the targeting relationship between miR-449a and KDM3A. Then, MCF-7 cells were divided into five groups for transfection: control group, negative control (NC mimic) group, miR-449a mimic group, miR-449a mimic+pcDNA3.1 group, miR-449a mimic+pcDNA3.1-KDM3A group, CCK-8 method, Transwell chamber and cell scratch assay were used to detect MCF-7 cell proliferation, migration and invasion in each group, Annexin V-FITC/PI double staining method and Western Blot were used to detect MCF-7 cell apoptosis and the expression levels of apoptosis-related proteins caspase 3, caspase 9, B cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax) in each group. Results Compared with adjacent normal tissues and MCF10A cells, the relative expression of miR-449a in breast cancer tissues and several cell lines MDA-MB-231, BT-474, MCF-7 and SKBR-3 was decreased, while the relative expression of KDM3A protein was increased (P<0.05); there was a targeting relationship between miR-449a and KDM3A, and miR-449a negatively regulated the expression of KDM3A; Compared with the control group and the NC mimic group, the proliferation activity of the miR-449a mimic group was decreased at 24 h, 48 h and 72 h at the same time, the number of cell migration and invasion was reduced, the scratch healing rate was reduced, and the cell apoptosis rate was increased, the relative expression of intracellular apoptosis-related proteins Caspase3 and Caspase9 and pro-apoptotic protein Bax was upregulated, and the relative expression of anti-apoptotic protein Bcl-2 was downregulated (P<0.05); Compared with the miR-449a mimic group, miR-449a In the mimic+pcDNA3.1 KDM3A group, the proliferation activity increased at the same time, the number of cell migration and invasion increased, the scratch healing rate increased, the cell apoptosis rate decreased, and the relative expression of Caspase3, Caspase9 and Bax proteins in the cells was downregulated, while the relative expression of Bcl-2 protein was upregulated (P<0.05). Conclusion miR-449a may inhibit the proliferation, migration and invasion of breast cancer cells and promote their apoptosis by targeting and inhibiting the expression of KDM3A, thereby playing an anti-breast cancer role