The anatomy reports indicated that atherosclerotic lesions were located on the inner wall,or both inner and out-er walls of the sinus of the carotid bifurcation in the many cases.This phenomenon was not explained by the hypothesis about the low wall shear stress in the traditional Y-AHCB model.In this paper,in the modified model of″average human carotid bi furcation(AHCB)″the three-dimension field in carotid sinus under steady flow conditions was studied using flow visualization techniques and laser-Doppler anemometer(LDA).The effects of the shape of branches on the flow separation and were observed.A new separation zone along inner wall of sinus was found.It can be used to explain plaque distribution on the inner wall of sinus.The modified model was proved to be a more rational model for hemodynamics and atheosclerosis study in human carotid bifurcation.