The phase transformation of different polymorphs in zirconia is very important for the processing and mechanical properties of zirconia ceramics. In this work, thermodynamic description of ZrO 2 -MgO system is investigated using the related thermodynamic parameters. Special attention is paid to the calculation of the Gibbs free energy change between tetragonal and monoclinic phases in ZrO 2 -MgO, namely t ! m phase transformation driving force, as a function of composition and temperature. Furthermore, in 8.7 mol% MgO-ZrO 2 , the equilibrium temperature between tetragonal and monoclinic phases, T 0 , was obtained as 1107.8 K and martensitic transformation start temperature (M s ) for t ! m of this ceramic with a mean grain size of 1.15 mm was also calculated to be as 737.7 K, which is in good agreement with experiment one of 689.5 K with 7.0% residual.