Cancer stem cells (CSCs) are maintained by inflammatory cytokines and signaling pathways. Tanshinone IIA (Tan-IIA) possesses anti-cancer and anti-inflammatory activities. The purpose of this study is to confirm the growth inhibition effect of Tan-IIA on human breast CSCs growth in vitro and in vivo and to explore the possible mechanism of its activity. Human breast CSCs were enriched and expanded under serum-free mammosphere culture condition, and identified through mammosphere formation, toluidine blue staining, immunofluorescence staining, and flow cytometry analysis of stemness markers of CD44/CD24 and ALDH, and tumorigenecity in vivo; the growth inhibition effect of Tan-IIA on human breast CSCs in vitro were tested by cell proliferation and mammosphere formation assays; inflammatory signaling pathway related protein expression in response to Tan-IIA, IL-6, STAT3, phospho-STAT3 (Tyr705), NF-κBp65 in cytoplasm and nucleus and cyclin D1 were evaluated with Western blotting; the growth inhibition effect of Tan-IIA on human breast CSCs growth were tested in vivo. A useful model of human breast CSCs for researching and developing the agents targeting CSCs was established. After Tan-IIA treatment, cell proliferation and mammosphere formation of CSCs were decreased significantly; the expression levels of IL-6, STAT3, phospho-STAT3 (Tyr705), NF-κBp65 in nucleus and cyclin D1 proteins were decreased significantly; the tumor growth and mean tumor weight were reduced significantly. Tan-IIA has the potential to target and kill CSCs, and can inhibit human breast CSCs growth both in vitro and in vivo through attenuation of IL-6/STAT3/NF-kB signaling pathways. J. Cell. Biochem. 114: 2061–2070, 2013. © 2013 Wiley Periodicals, Inc.