Mega-crystalline calcite (m-CC) breaks apart easily during calcination, and is converted to CaO with difficulty due to its thermal decomposing characteristics that necessitate large heat consumption. To overcome this problem, m-CC was calcined using either a microwave furnace or an electric furnace fo 15 min and 30 min at 950 ℃. Following 15 and 30 min calcinations of m-CC, almost all (96.6 wt%) and all (100%) of the calcite was calcined to CaO in the microwave furnace, compared to only 11.3 wt% and 11.0 wt% in the electric furnace, respectively. The excellent conversion rate in the microwave furnace was attributed to its internal heating method. Therefore, calcination of m-CC to CaO was optimized in the microwave furnace at 950 ℃ for 30 min. The results demonstrated the potential for the microwave furnace calcination of m-CC, previously known as a non-calcinable material, as a material source for CaO, Ca(OH)2 and CaCO3.