Abstract: This study investigated the inhibitory effect and mechanisms of cryptotanshinone (CPT) on tamoxifen resistant cell MCF7-TAMR. The inhibitory effect of CPT on the viability of MCF7-TAMR cells was evaluated using the MTT assay. We found that CPT significantly inhibited the growth of MCF7-TAMR cells in a dose- and time-dependent manner. The half inhibitory concentration (IC₅₀) is 15.14 ± 2.82 μmol·L^-1 at 24 h. CPT induced cell cycle arrest of MCF7-TAMR cells at G0/G1 phase, and promoted apoptosis of MCF7-TAMR cells by upregulating intracellular levels of reactive oxygen species (ROS). Transwell results showed that CPT significantly inhibited the migration of MCF7-TAMR cells. Furthermore, CPT decreased the CD24^-/lowCD44⁺ cell population in MCF7-TAMR cell-derived microspheres. Western blot results showed that CPT effectively inhibited the phosphorylation of estrogen receptor α (ER-α), and reduced the expression of phosphatidylinositol 3-kinase (PI3K-p85), serine-threonine protein kinase (Akt) and multidrug transporter ATP-binding cassette superfamily G member 2 (ABCG2). These results showed that CPT can induce cell apoptosis, cause cell cycle arrest, inhibit cell migration and inhibit ER-α phosphorylation, inhibit PI3K/Akt signaling pathway, reduce the number of CD24^-/lowCD44⁺ cells and the expression of ABCG2, overcome cell drug resistance.