OBJECTIVE To explore the mechanism of oridon in（Ori）reversing the drug resistance of breast cancer cell MCF-7 to fulvestrant （Ful）. METHODS Ful-resistant breast cancer cell strains MCF- 7/Ful were induced and constructed in vitro . The relative cell viability of MCF- 7 cells and MCF- 7/Ful cells was detected by MTT assay ，inhibitory rate of Ori to MCF- 7/Ful cells was also detected. CompuSyn software was used to analyze the synergistic effect of Ori and Ful. MCF- 7/Ful cells were randomly divided into blank control group ，Ful group （5 μmol/L），Ori group （8 μmol/L），Ful（5 μmol/L）+Ori（8 μmol/L）group. The phosphorylation of phosphatidylinositol 3 kinase（PI3K）/protein kinase B （Akt）signaling pathway related protein in each group was detected by Western blot. MCF-7/Ful cells were used to prepare drug resistance model of transplanted tumor in nude mice，and they were randomly divided into blank control group ，Ful group （80 μmol/g），Ori group （50 μmol/g），Ful（80 μmol/g）+ Ori（50 μmol/g）group. The tumor weight and tumor inhibition rate were calculated ，to verify the reversal effect of Ori and Ful in vivo. RESULTS MTT assay showed that when Ful ≥10 μmol/L，the relative cell viability of MCF- 7/Ful cells was significantly higher than that of MCF- 7 cells（P＜0.05），and the drug resistance was significantly enhanced ；Ori had a significant inhibitory effect on MCF- 7/Ful cells ，the inhibition rate of Ori combined with Ful to MCF- 7/Ful cells was significantly increased （P＜0.05 or P＜0.01），and the effect of reversing drug resistance was significantly increased . The results of Western blot showed that compared with Ful group ，the phosphorylation levels of PI 3K and Akt protein in Ful+Ori group were significantly decreased （P＜0.05 or P＜ 0.01）. In vivo results showed that the tumor volume and mass of Ful+Ori group were significantly decreased ，and the tumor inhibition rate was （63.90±4.11）％，which was significantly higher than that of Ful group （P＜0.01）. CONCLUSIONS Ori can reverse drug resistance of breast cancer cell MCF- 7 to Ful ， and this effect may be through regulating PI 3K/Akt signaling pathway.