TEM in situ continuous observations were made of the tetragonal (t) to monoclinic (m) transformation of ZrO2(2Y) in pure ZrO2(2Y) and Al2O3+25 vol % ZrO2(2Y) ceramics. It was found that there were two morphologies of the m-phase after transformation of the t-grains : N-shaped plate clusters and parallel laths. The parallel lath morphology is considered to be the result of lateral growth of the m-plates. The lower the local Y2O3 content, the less will be the stability of the t-phase, and then the larger will be the possibility of the lateral growth of the m-plates. This, in turn, facilitates the transition from N-shaped to parallel lath morphology and increases the completeness of the transformation.