The Discrete Element Method (DEM) is used to investigate the compaction of ceramic powders by focusing on the role of primary particle and aggregate sizes. Hard aggregates of primary particles are represented as a collection of spherical particles bonded together by solid necks, which may break during compaction. Numerical simulations are performed to investigate the effect aggregate microstructure both on compaction and on the green strength of the ceramic compact. Tensile green strength originates only from van der Waals adhesion, without any action of a binder. It is shown that in such conditions, the green strength is inversely proportional to the size of the primary particles. The size of aggregates also plays a role, with smaller aggregates leading to larger green strength. The results of the simulations are compared to experimental data obtained on dioxide uranium aggregated powders, confirming the particle size effect. (C) 2012 Elsevier Ltd. All rights reserved.