Abstract: Objective To observe the effect of octylphenol(OP) on cell proliferation, cell cycle and cyclin expression of MCF-7 breast cancer cells. Methods Using MTT test, flow cytometry, immunocytochemistry and RT-PCR to observe the changes of cell proliferation, cell cycle, apoptosis, CDK2,CDK4 and Cyclin D1 mRNA expression, Cyclin D1 protein expression of MCF-7 cells. Results When MCF-7 breast cancer cells were treated with 4,8 and 16μmol/L OP on 72h, the proliferation ratio of MCF-7 was 107.31%、168.06% and 62.00% respectively, MCF-7 cell at G₀ / G₁ were (52. 46 ±6. 67) %、(50. 19 ±7. 39) % and (67. 31 ±5. 47) % respectively, while cont rol group was (55. 27 ±7. 53) %. The apoptosis ratio of MCF-7 was (4. 84 ±1. 12) %, (6. 48 ±1. 36) % and (19. 26 ±3. 57) % respectively, and the control group was (5. 56 ±1. 125) %. The expression of CDK2, CDK4 and Cyclin D1 mRNA significant decreased when MCF27 breast cancer cells were t reated with 16μmol/ L OP, the expression of Cyclin D1 protein decreased too. Conclusion 　The proliferation of MCF-7 breast cancer cells increases when t reated with 4, 8μmol/ L OP, but it decreases when t reated with 16μmol/ L OP. These different effect s on MCF-7 cells maybe related to the expressive regulation of CDK2, CDK4 and Cyclin D1.