The author reports a method for inspection of optical disks based on diffraction measurements. An optical disk has very fine grooves, so that light diffracted from it is spread over large angles, and the change of inclination factor related to Fresnel's coefficients at grooves cannot be ignored. To predict diffraction patterns, Fraunhofer's integral has to be modified. In actual fabrication the groove form may deviate from the ideal rectangular cross section. Moreover, multiple reflections from substrate surfaces can also affect the diffraction patterns. A method is presented to measure the power spectrum of a light beam scattered or diffracted by an optical disk and to recover parameters (pitch, width, and depth of grooves) of optical disks from these power spectra. The wide-angle diffraction problem is solved by taking effects of the inclination factor into account. The effects of groove form and multiple reflections on the diffraction patterns are also included in the theory. To estimate the accuracy of this method, the optical disks were also examined by the SEM technique. The two methods yield results agreeing to 1.9% for the pitch, 5.6% for the width, and 8.6% for the depth.