Abstract: Both radial component and axial component are the favorite testing factors in the magnetic flux leakage testing(MFLT). Equivalent magnetic dipole model was taken to analyze the distribution feature of magnetic flux leakage of defects, demonstrating the fact that radial component decays more rapidly than axial component. The flux leakage due to steel pipe defects with various defect depth(10% to 40%) and width(0.5 mm to 20 mm) was calculated using finite element method. The sharpness angle of the notch was also taken into consideration in the calculation. The critical point, where the amplitude of radial component is equal to that of axial component, transits with various parameters of defects. The tendency of shift of critical point is discussed for the different cases. The effects of lift-off of sensor and defect parameters on amplitude of components of flux leakage were studied from sensor design. The conclusion is drawn that critical point can be used to determine which component would be pick out by one dimensional sensor in designing MFLT equipments.