Zinc oxide is a widely used material in various applications in electronic, optic, and spintronic fields, in particular. The control of the final properties of ZnO requires the mastering of the final microstructure. To achieve this goal, the grain growth of ZnO has been examined as function of the sintering conditions, in particular in using a specific microwave sintering method. In order to get nano-sized ZnO powder as a starting material, a liquid route was implemented. The latter is based on the direct precipitation of a zinc oxalate solution. After thermal treatment, pure ZnO powder was obtained with a very narrow grain size distribution, centered at around 20 nm. The sintering of this powder was then carried out in conventional and microwave furnaces. While an important grain growth occur during the conventional sintering, it is shown that microwave sintering allows us to maintain the grain size at the nano-metric scale. (C) 2010 Elsevier B.V. All rights reserved.