In the coal water slurry (CWS) industry, it is essential to achieve high packing density of product particles in order to obtain a high concentration without affecting the flowability. In this study, a method based on an index E is proposed for evaluating the packing density of particles in CWS with given particle size distribution (PSD) and tested on bituminous and lignite coal. These two types of coal were ground and mixed in different proportions to obtain different packing densities. The experimental results show that the CWS concentration increased with E, implying the good applicability of the proposed method. By calculating E for different PSDs, it was found that the packing got closest when the parameters in Rosin-Rammler equation and Alfred equation were 0.75-0.85 and similar to 0.5, respectively. In addition, the packing density for unimodal PSD is generally lower than that for multimodal PSD. In contrast, the average sizes of the mixed samples did not affect the packing status in CWS and the product concentration consistently. With some adjustments, the proposed method can also be used to evaluate the tapped packing density of dry coal powders, with some applicability. (C) 2015 Elsevier B.V. All rights reserved.