A blood flow analysis system using MRI is presented. In order to measure the blood flow of a vessel, a vessel region of interest in the magnitude image is first located. Then contours of the vessel are detected and are superimposed on the phase contrast image from which a set of quantitative flow parameters can be calculated. Comparing to the traditional segmentation in blood flow analysis proposed by the research group in Stanford University [1], the proposed method is more accurate in vessel segmentation and performs better blood-flow measurement in motional images caused by heart beating and respiration.