The abnormal phenomena in the transmitted light passing through α-LiIO3 single crystals under the application of a DC field parallel to the c-axis have been further investigated by means of the direct projection method, the optical Fourier transfor-mation method and the Schlieren method. The change in the spacial frequency spectrum of a z-cut crystal plate with the increase of the applied field intensity was examined. It was observed that the diffraction efficiency of the transmitted light shows a linear dependence on the applied voltage, but is independent of the polariza-tion of the incident light. It was also found that the light polarization is not changed by the action of the DC field and the effect of absorption is negligible. The relaxa-tion process of the change in diffraction efficiency with time was recorded quantita-tively. A tentative explanation of these phenomena has been given.