The temperature dependence of composition, unit cell parameters, thermal expansion coefficients and microstructure during complete thermal decomposition of calcite has been investigated by in-situ high-temperature X-ray powder diffraction. The respective X-ray diffractograms were obtained from 675 to 800 °C at 25 °C intervals. Results indicate that the thermal conversion of calcite to calcium oxide is initiated at a slow rate and rapidly decomposed beyond the temperature of 750 °C. During the period of thermal decomposition, positive and negative expansion of the unit cell is detected parallel to c-axis and perpendicular to a-axis, respectively. A substantial reduction in unit cell volume by 69% is observed during the conversion from trigonal calcite to cubic calcium oxide. It is also found that the rapid composition change and unit cell volume reduction simultaneously accounted for polynomial behavior of thermal expansion coefficients of calcite. Both crystallite size and lattice strain of calcite are increased simultaneously during thermal decomposition. A considerably higher lattice strain is observed for calcite compared to calcium oxide at all decomposition temperatures. The uniform arrangement of small cubic unit cells in newly materialized calcium oxide is accounted for moderate crystallite size and lowest lattice strain at the completion of thermal decomposition. Display omitted •Decomposition of calcite is initiated slowly and rapidly decomposed above 750 °C.•CO2 gas emission catalyzed the conversion of trigonal CaCO3 (calcite) to cubic CaO.•Polynomial behavior in thermal expansion coefficients is observed for calcite.•Lattice strain and crystallite size of calcite increased rapidly with temperature.•Stabilization of CaO is achieved by moderate crystallite size and lowest lattice strain.