Microstructural evolution and grain growth in 3Y-TZP ceramics have been examined using two nano-sized 3Y-TZP powders, calcined at different temperatures, derived from the same hydroxide powder. The green bodies prepared from both powders showed two sets of porosity: inter-agglomerate and intra-agglomerate. The intra-agglomerate pores are related to initial crystallite size; the size of the inter-agglomerate pores are governed by the pressure applied to the green bodies. Due to the nano-sized dimensions, the intra-agglomerate porosity has little effect on the final density and grain size. In contrast, the large inter-agglomerate pores play a major role on both the sintering process and achievement of final density. During sintering, the grains initially grew to approximately 100 nm, reaching the intermediate stage of sintering; the grain growth rate then became very much reduced before reaching the final stage of the sintering process. At this point, significant grain growth took place, possibly controlled by the solution drag mechanism. The activation energy for the grain growth was determined to be 352 kJ/mol.