Light diffraction patterns from LiKSO4 single crystals and their changes with temperature were observed. These crystals were also observed under an optical microscope. It is concluded that the optical phenomena described in Reference [1] are caused by optical twins which are arranged in phase gratings with wave vectors perpendicular to c axis in LiKSO4. Diffraction patterns show abrupt changes at 435℃, -73℃ and -84℃, caused by structural phase transitions at these temperatures.