The powder rheometer is a commonly used device for assessing the bulk flow performance of pharmaceutical powders. Discrete element simulations were performed to determine the effects of particle properties such as size, shape, size distribution and friction on the force and torque on the impeller blade in a powder rheometer. DEM simulations are well suited for such a study as they can isolate the impact of each particle property on the bulk powder behavior. The results can then be used to guide the 'particle engineering' of pharmaceutical powders to meet specific performance targets. The operation of the FT4 Freeman powder rheometer system was simulated using the discrete element method (EDEM(TM) from DEM Solutions). The effects of various particle properties (size, shape, friction, etc.) were examined using the force and torque on the impeller blade as the key performance indicators. The effect of particle size (mean, distribution) on the mixing torque/force was small and the use of a pre-conditioning step also had minimal impact. As the particle aspect ratio was increased from 1.0 (perfect spheres) to 2.0, both the torque and force values also increased (max increase of similar to 40%). Increasing the rolling friction of spherical particles produced similar results as the large aspect ratio particles. Increased particle-particle friction caused a larger increase in the measurements (max increaseof similar to 60%) in comparison with increased particle-vessel and particle-impeller friction (max increase of similar to 20%). Experiments with glass beads were also performed and were used to validate the simulations. (C) 2010 Elsevier Ltd. All rights reserved.