In this paper, the Monte Carlo simulation was used to simulate the diffuse reflectance of four breast tissues, which includes the normal breast tissue and three diseased human breast tissues (the breast fibroadenoma, the mastopathy and the breast cancer). The simulation was made in the wavelength of 633nm. To verify the simulations, experiments were done to measure the diffuse reflectance. The measurement system is made up of a CCD and phantom of tissue which made from aqueous suspension of Intralipid-20% and ink. The relation between the light intensity and the distance from the light source were obtained after the data processing. The results of the simulation and the experiments both showed that the differences of the diffuse reflectance in the four kinds of tissues were significant. This method may be useful in the clinical diagnosis of the breast diseases.