In this work the influence of isothermal sintering time on the microstructural development of ZrO2-Al2O3 composite was studied. Powder mixture of ZrO2 containing 20 wt% Al2O3 was prepared by milling, compaction and sintering at 16000C, in air. The isothermal sintering time at 16000C was varied between 0 and 1440 min. The sintered samples were characterized in terms of phase composition and relative density. Their microstructures were characterized by grain size distributions and average grain size. These results were evaluated using the classic grain growth equation as a function of time, determining the grain growth exponent of these materials. Furthermore, the microstructural aspects were related to the mechanical properties (Vicker’s hardness and fracture toughness) of these composites.