Based on the Al2O3-ZrO2-SiO2 system, the conventional pre-sintering samples were reheated by microwave to investigate the influence of microwave to the thermodynamic equilibrium and phase equilibrium relationships. It was found that microwave can accelerate transitions to thermodynamic equilibrium. The phase equilibrium of the samples prepared by microwave heating was identical with the traditional phase diagram. In addition, the amount of tetragonal zirconia in the prepared samples remained constant as the microwave heating time prolonging. All of the observed phenomenon can be explained by microwave nonthermal effects.