In this paper, the diffraction bands and spots experimentally observed when a laser beam passed through a KIO3 single crystal are reported. The bands and spots are formed by the diffracted beam, the plane of polarization of which rotates 90°with respect to that of the incident beam. The temperature dependence of the diffracted light was investigated in the region from room temperature to 240℃. It was found that when T=72℃ the intensity increased abrupty, when T=212℃ it decreased ;altnost to zero, and when T>212℃ no diffracted light was observed. The two temperatures, 72℃ and 212℃, determined from the anomaly of the intensity dependence, are in agreement very well with the phase transition temperatures of KIO3 crystals reported in the literature.