The moire fringe′s amplitude distribution is analyzed by using the frequency domation analyses of Fresnel diffracting.The formula of small angle measurment using the grating pair composed of the same gratings is gained.The formulas gained by frequency domain analysis are the same with those gained by geometrical method.The results show that moire fringe is composed of lots of vertical small fringes.To measure the small angle directly by the change of the width of the moire fringe,a initial angle is needed,and it′s value is decided by the magnification of detector′s optical system,the angle measuring precision,the size and the resolution of the detector.