In this article, we investigate the hydrogen storage capacity of Mg-decorated gamma-graphyne (Mg-G) based on DFT calculations. Our results indicate that an external electric field can effectively prevent Mg atoms aggregating on gamma-graphyne sheet. The Mg-G, after electric field (F = 0.05 V/nm) treatment, can store up to ten H-2 molecules and the hydrogen storage capacity is 10.64 wt%, with the average adsorption energy of 0.28 eV/H-2. Our calculations demonstrate that Mg-G is a potential material for hydrogen storage with high capacity and might motivate active experimental efforts in designing hydrogen storage media. (C) 2016 Elsevier B.V. All rights reserved.