Decomposition kinetics of magnesite was investigated by using thermogravimetric (TG) analysis. The activation energy (E) and pre-exponential factor (lgA) were calculated from Kissinger and Ozawa-Doyle equations, and the peak value of derivative thermogravimetry (DTG) data was used as the maximum temperature. The calculated E and lgA were 211.55 kJ/mol and 11.07 s^-1, respectively. The reaction mechanism was further discussed by using single-TG curve, multivariate no-linear regressions and Malek method, respectively. The obtained results indicate that the reaction was a three-dimensional phase boundary reaction (R3). The kinetic equation was as follows: dα/dT=(10^11.07/β)·e^{-211.55×103/(8.314×T)}·3(1-α)^2/3.