The behaviour of isothermal phase transformation, during ageing at 250-degrees-C, in 2Y-TZP and related microstructural development, were investigated. The mode of microstructural development due to isothermal phase transformation was greatly dependent on grain size and the density of the sintered body. Isothermal phase transformation and related microstructural changes began at the specimen surface, near the pores, where the change of strain free energy for a t-m transformation was small and water vapour was easily contacted. The transformed region showed porous microstructures, which appeared due to the formation of cracks upon isothermal transformation.