The strains generated in a reaction vessel of hydrogen storage alloys and the packing ratio distribution inside the vessel were measured in order to analyze the effects of packing on stress. More specifically strains generated on the vessel's surface were measured when hydrogen is repeatedly absorbed and desorbed by the packed bed in the reaction vessel. The amount of deformation, local packing ratios and relative particle volumes in the vessel were also measured after repeated hydrogen absorption-desorption. As absorption-desorption was performed repeatedly, agglomeration regions where the value of the local packing ratio was around 0.6 were formed, and particularly strong stress was generated in these regions, causing deformation. More hydrogen packing causes agglomeration regions to form over a wider area. Since alloys are pulverized by repeated absorption-desorption, and concentrate in the lower parts of the vessel, agglomeration regions are also formed in the lower parts. Our experiments also revealed that the resulting agglomeration regions have a packing ratio of about 0.6. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.