The decomposition kinetics of magnesite has been investigated using thermal analysis. The effect of two important parameters namely temperature and particle size are investigated. Isothermal TG experiments were carried out at temperatures of 873 K, 923 K, 973 K and 1023 K, ensuring that the reaction did not go beyond the magnesite decomposition stage. The reduced time plots have been used in the preliminary identification of reaction mechanisms. Accordingly all the kinetic data were examined with respect to the topochemical contracting sphere model. The linear plots confirmed the assumption. The activation energy values were in the range 95-104 kJ/mol.