Abstract: OBJECTIVE To investigate the possible mechanism by which octylphenols (OP) induces apoptosis and inhibits proliferation of human breast cancer cells. METHODS MTT assays and flow cytometry analysis were employed to examine cell proliferation and cell cycle distribution, respectively. The expression levels of cell cycle associated protein p27Kip1 and CyclinE were determined by Real time PCR and Western blot analysis. The protein level of PAK4, an apoptosis inhibitor, was also analyzed. RESULTS OP significantly inhibited the growth of MCF-7 cells and caused G0/G1 cell cycle arrest in MCF-7 cells. In addition, after treated with OP, the percent of cells in S phrase was reduced. OP treatment decreased the expression of CyclinE at both mRNA and protein level in a dose-dependent fashion. However, elevated expression of p27Kip1 was observed after OP treatment. CONCLUSION OP may inhibit the proliferation of MCF-7 cells by regulating the expression cell cycle associated proteins.