The effect of particle size on hydrolysis properties of hydrogenated Mg3La was investigated through fixing the grain size of in-situ formed LaH3 and MgH2 from hydrogenated Mg3La. The hydrolysis rate and hydrogen yield were affected by its particle size. The hydrogenated Mg3La with smaller particle size of [<12] mu m had a higher hydrolysis yield of 863 ml g(-1) (7.70 wt.%) hydrogen. The surface area and defect of samples were increased through reducing the particle size and thus promoted the complete hydrolysis. Reducing the particle size is an effective and simple method to improve hydrolysis properties of magnesium hydrides-based materials. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.