Thermal decomposition of dolomite was investigated using DTA-TG and TEM. There have two obvious endothermic valley and the first one is narrow and sharp, the second endothermic valley is wide and flat. Between the two valley there is a temperature difference about 15-20°C and no decomposition in this region. Before the decomposition of dolomite some spherical particles formed on the surface of mineral grain. The first endothermic valley formed when these particles started to decompose, and the remaining material decomposed into the second endothermic valley.