Objective：To investigate the expression of miR-18a in breast cancer tissues and to explore the regulative effects of miR-18a antisense oligonucleotide（ASO） on the invasion and migration of breast cancer cells in vitro. Methods：The expression of miR-18a in 108 breast cancer tissues and their adjacent breast tissues were detected by real-time quantitative PCR method. After trans－fection with miR-18aASO，the biological effects of miR-18a on breast cancer cells was measured by Transwell assay and wound healing assay and invasion-related protein expression was analyzed by Western blot. Results：miR-18a was found to be overexpressed in 50.93%（55/108） of the breast cancer cases（t=28.68，P=0.000）. miR-18a expression in breast cancer cells（transfection with miR-18a ASO） was significantly less than that of control group（F=321.67，P=0.001；F=563.28，P=0.000）. Transwell and wound healing as－say results showed that the invasion and migration ability decreased greatly after the transfection with miR-18a ASO（P=0.000），fur－thermore，down-regulation of miR-18a resulted in obvious inactivation of matrix metalloproteinase（MMP）2 and MMP9（P=0.000）. Conclusions：miR-18a is overexpressed in human breast cancer. Reducing the expression of miR-18a can effectively inhibit the inva－sion and migration of breast cancer cells. miR-18a may become a new target for the regulation of invasion and migration in breast can－cer.