In present study, the thermal behaviour of calcined gypsum specimens at temperature controlled laboratuar farin in the temperature range 120-1000 degrees C in atmospheric pressure by means of X-ray difractometer and scanning electron microscopy is examined. In this investigations, powder gypsum and calcined powder gypsum at different temperatures (120, 160, 180, 200, 250, 600 and 1000 degrees C) were used. Scanning electron microscopy observations and X-ray diffractometer results show that the transformation to bassanite (calcium sulfate hemihydrate) begins at ca. 1200 degrees C, the transformation to anhydride complete at ca. 600 degrees C. Differential thermogram result was pulsed at these transformations.