The non-random distribution of atherosclerosis was related to local hemodynamic environment. The stabilization of endothelium was important in this process. We studied the effect of magnitude of shear stress on proliferation of endothelial cells. It was shown the proliferation of endothelial cells was inhibited by shear stress, and was related to the magnitude of shear stress. A parallel plate sudden-expansion flow chamber was constructed, and the effect of flow pattern was also studied. It was shown the inhibition effect produced by shear stress was decreased in this chamber.