OBJECTIVE To study in vitro inhibitory effects of realgar nanoparticles on breast cancer stem cells. METHODS Human breast cancer MCF- 7 parent cells were selected as subjects and cultured by serum-free culture to obtain breast cancer stem cells. Using adriamycin （1 mg/L）as positive control ，same concentration of water-processed realgar as reference ，the effects of realgar nanoparticles on the proliferation of MCF- 7 parent cells and stem cells were detected by CCK- 8 method. The effects of realgar nanoparticles on the formation of mammosphere ，the ability of differentiation ，migration and invasion ，the proportion of CD44+/CD24－ subgroup in breast cancer stem cells were detected by mammosphere formation and differentiation experiment ， scratch experiment ，Transwell invasion experiment and flow cytometry. Western blot assay was used to detect the expression of proteins related to epithelial mesenchymal transformation pathway （E-cadherin and vimentin ） in breast cancer stem cells. RESULTS The survival rates of MCF- 7 parent cells and stem cells （except for breast cancer stem cells in both 1 mg/mL groups ）in 1，5，10，40，60，80 mg/L groups of water-processed realgar and realgar nanoparticles were significantly lower than blank control group（P＜0.01）. The number of mammosphere （＞20 stem cells ）in 1，2.5，5，10 mg/L groups of water-processed realgar and realgar nanoparticles was significantly lower than blank control group （P＜0.01）；the volume of mammosphere decreased and the differentiated adherent cells decreased ；the healing rate of wound ，relative invasion rate （except for water-processed realgar 1 mg/L group）and the proportion of CD 44+/CD24－ subgroup were significantly lower than blank control group （P＜0.01）. The expressions of E-cadherin in 2.5，10 mg/L groups of water-processed realgar and realgar nanoparticles was significantly higher than blank control group ，and the expressions of vimentin was significantly lower than those in blank control group （P＜0.01）. The above effects of realgar nanoparticles were generally better than those of water-processed realgar with the same mass concentration （P＜ 0.01）. CONCLUSIONS Compared with water-processed realgar with the same mass concentration ，realgar nanoparti cles can significantly inhibit the proliferation of breast cancer stem cells， the formulation and differential ability of mammo- sphere，and reduce the proportion of CD 44+/CD24－ subgroup. The effect may be associated with the inhibition of migration and invasion of breast cancer stem cells by inhibiting the expression of proteins related to epithelial mesenchymal transformation pathway.