In situ decarbonation kinetics of calcite were investigated at high temperatures, up to 900 °C, using synchrotron radiation powder X-ray diffraction. The sequence of X-ray diffraction spectra reveals that calcite begins to thermally decompose into lime (CaO) and CO2 at 800 °C and ambient pressure. The decarbonation degree gradually increases with temperature, and calcite completely transforms into lime at 900 °C. The kinetic analysis of the isothermal data using an Avrami model involving nucleation and growth yields the values for the decarbonation rate and reaction order. Our results indicate that the decarbonation rate increases from 2.89 × 10–4s–1 to 3.48 × 10–3s–1 with elevated temperature from 840 to 880 °C, showing a positive temperature dependence on the reaction rate. The calculated Avrami exponent (n) values between 1.35 and 2.38 suggest that the thermal decomposition of calcite should be mainly dominated by homogeneous nucleation and CO2 diffusion-controlled growth. In natural carbonate fault rocks, the decarbonation of CaCO3 caused by frictional heating may be strengthened by the action of high shear velocity. In addition, the resulting ultrafine powder and CO2 pressurization can remarkably reduce the friction coefficient between two fault planes, which further leads to carbonate fault weakening. The yielding result will be conductive to better understanding the role of decarbonation of calcite in some active fault zones.