Objective To investigate the functional role of 25 microRNAs in breast cancer,and to find new tumor suppressor microRNAs that may serve as specific targets of new gene therapies. Methods Twenty-five microRNAs expression vectors were constructed and stably transfected into mouse mammary tumor cells 4To7 by Lipofectamine2000. Cells were selected with G418 and sorted by Flow cytometry. The cells in logarithmic phase were collected and 2×10⁵ cells/mouse was inoculated into BALB/c mice via tail vein. Lungs were harvested 14 days after tumor cell inoculation, and the number of metastasis foci was counted. Results Mice inoculated with mir-449a-expressing 4To7 cells via tail vein developed reduced lung metastases compared with mice inoculated with negative control cells. Mice inoculated with mir-1935-expressing 4To7 cells via tail vein developed increased lung metastases compared with mice inoculated with negative control cells. Other twenty-three microRNAs neither promoted nor inhibited lung metastases of breast cancer. Conclusions Two of twenty-five microRNA were identified to be associated with breast cancer metastasis. MiR-449a may play a tumour suppressor role in the regulation of migration and metastasis in breast cancer. miR-1935 transgenic over-expression promoted tumor growth and metastasis.