Understanding the relationship between the applied stress and the behaviour of packings of breakable granular particles is required in many chemical engineering applications. A pharmaceutical tablet is an example where a composite packing (a packing containing a mixture of several kinds of particles) is compacted in the process of tablet formation. In this work, the unidirectional compaction and the breakage processes in composite packings formed from a mixture of breakable needle-shaped crystals and elastic spherical filler particles were studied using a Discrete Element Method (DEM) simulation. The evolution of crystal size distribution and contacts topology with the increasing stress was computed for different relative size, volume fraction and elasticity of filler particles inside the packing. We show that crystals breakability in a packing is directly related to the number of contacts among particles and that it can be significantly influenced by an appropriate choice of added filler particles. The results can be used for the development of mathematical models that describe the pharmaceutical tablet production and other processes involving composite packings of fragile particles. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.