MicroRNAs have key roles in tumor metastasis. Here, we describe the regulation and function of miR-34a and miR-34c (miR-34a/c) in breast cancer metastasis. Expression analysis verified that miR-34a/c expression is significantly decreased in metastatic breast cancer cells and human primary breast tumors with lymph node metastases. Overexpression of miR-34a/c could inhibit breast cancer cell migration and invasion in vitro and distal pulmonary metastasis in vivo. Further studies revealed that Fos-related antigen 1 (Fra-1 or Fosl1) is a downstream target of miR-34a/c as miR-34a/c bound directly to the 3'untranslated region of Fra-1, subsequently reducing both the mRNA and protein levels of Fra-1. Silencing of Fra-1 recapitulated the effects of miR-34a/c overexpression, whereas enforced expression of Fra-1 reverses the suppressive effects of miR-34a/c. Moreover, significant downregulation of miR-34a in metastatic breast cancer tissues was found to be inversely correlated with Fra-1 expression. Our results demonstrate that miR-34a/c functions as a metastasis suppressor to regulate breast cancer migration and invasion through targeting Fra-1 oncogene and suggest a therapeutic application of miR-34 in breast cancer.