The energetics of martensitic transformation in ZrO2is studied using a thermodynamic approach, with particular reference to Al2O3–ZrO2composites. The different characters of three types of transformation‐toughened ceramics are analyzed, and several factors affecting thet→mtransformation in Al2O3–ZrO2composites are discussed. The expression of transformation temperature dependence on particle size is derived and has good agreement with experimental results. The energetics concerned with nucleation of martensitic transformation is also di