Based on the anisotropic diffraction theory of phase gratings of Stepanov et al., using the method developed by Gu and Li, we have made a detail calculation of the effect of anomalous diffraction of a laser beam passing through a cube of LiNbO3: Fe crystal. Fourier transform of the chaotic destribution of electric charge excessively accumalated in small regions shows the existence of infinite number of phase gratings. The diffraction pattern is determined by the diffraction efficiency of these gratings. The results of calculation show that the two lobes on both sides of the central spot the shapes of which are determined by the lease effect can be interpreted. Theory and experimental data fit fairly well.