In ref.1 a theory has been formulated for treating the anisotropic modes (o→e) of diffraction observed in α-LiIO3 single crystals under a d. c. field. In the present article we consider the isotropic modes (o -o, e e) of diffraction observed in similar conditions with the same material. There are two kinds of diffraction bands: (1) those perpendicular to the c-axis and wide in angular extension, and (2) those inclined to the c-axis and limited in angular extension. For the former, our theoretical derivation yields the experimental findings that with the incident beam normal to the c-axis only very weak o → o and e →e bands may appear, while the o → o band remains weak but the intensity of the e → e band multiples as the angle between the incident beam and the c-axis decreases from π/2. For the latter, we took into account the grating effect of growth layers of the crystal enhanced by the enrichment of space charge. From the angular extension of an inclined band is about 8°, we estimated the deviation in parallelism between growth layers being no more than 2?and their average spacing about or larger than 5 μm. We also explained the diffraction spots reported in ref.4. The theory were given in a version more general than that in ref.1.