Objective:To investigate the growth effect of breast cancer cells by insulin. Methods:MCF-7 cells were exposed to the insulin at various concentrations(25,50,100,200 nmol/L) for 24, 48 and 72 h. The effect of insulin on cell proliferation was assessed by morphologic observing and colorimetric MTT methods. Results:When exposed to the insulin at 200 nmol/L for 72 h, the modality of MCF-7 cells had a significant change. The villus in micro lacuna of cytoplast decreased and mitochondrion expanded obviously. Insulin at different concentrations can significantly promote the cell proliferation, especially at the concentrations of 50, 100 and 200 nmol/L insulin for times of 24,48 and 72 h(P < 0.05 or P < 0.01). And it was showed that this effect was dose- and time- dependent. Conclusion:Insulin can stimulate the proliferation of breast cancer cells in a dose-and time-dependent way. It was indicated that the insulin was a factor to stimulate the proliferation of MCF-7 cells line.