Triple-negative　breast　cancer　(TNBC)　is　currently　the　most　malignant　subtype　of　breast　cancers　without　effective　targeted　therapies.　Mifepristone　(MIF),　a　drug　regularly　used　for　abortion,　has　been　reported　to　have　anti-tumor　activity　in　multiple　hormone-dependent　cancers,　including　luminal　type　breast　cancers.　In　this　study,　we　showed　that　MIF　suppressed　tumor　growth　of　the　TNBC　cell　lines　and　patient-derived　xenografts　in　NOD-SCID　mice.　Furthermore,　MIF　reduced　the　TNBC　cancer　stem　cell　(CSC)　population　through　down-regulating　KLF5　expression,　a　stem　cell　transcription　factor　over-expressed　in　basal　type　TNBC　and　promoting　cell　proliferation,　survival　and　stemness.　Interestingly,　MIF　suppresses　the　expression　of　KLF5　through　inducing　the　expression　of　miR-153.　Consistently,　miR-153　decreases　CSC　and　miR-153　inhibitor　rescued　MIF-induced　down-regulation　of　the　KLF5　protein　level　and　CSC　ratio.　Taken　together,　our　findings　suggest　that　MIF　inhibits　basal　TNBC　via　the　miR-153/KLF5　axis　and　MIF　may　be　used　for　the　treatment　of　TNBC.