A method for detecting phase and amplitude error of an alternating phase shifting mask is disclosed. In an exemplary embodiment, the method includes passing a collimated light beam through a pair of adjacent, phase shifted openings in the phase shifting mask. Then, the beam intensities of diffracted light passed through the pair of adjacent openings are recorded as a function of angular position. From the recorded beam intensities, an angle .alpha. is determined at which a first diffraction order and a second diffraction order occurs, wherein .alpha. represents a deviation from a pair of symmetrically distributed diffraction orders. In addition, a minimum intensity and a maximum intensity are also determined from the recorded beam intensities, wherein the phase error is calculated from the determined value of .alpha., and the amplitude error is calculated from the minimum intensity and said maximum intensity.