Ultrahigh pressure (UP) plays a crucial role in modifying structures and properties of functional materials. The effects of UP treatment (4 GPa) on phase transition and hydrogen storage properties of Mg12NiY alloys has been investigated at the temperature range of 800-1300 degrees C. The results show that the dimension of 18R-type long period stacking ordered (LPSO) structure in the Mg12NiY sample after UP treatment at 1300 degrees C is two orders of magnitude smaller than that in the as-cast sample. The hydrogen storage capacity, kinetics and cycle properties of Mg12NiY alloys are concurrently improved after UP treatment. The two-step reaction process is confirmed during hydrogenation process by combining cycle testing and in-situ transmission electron microscopy (TEM) observations. The reasons for high hydrogen storage properties are mainly related to three aspects: the increased volume fraction of high angle interfaces between LPSO phase and matrix, the reduction of hydrogen diffusion distance, and the low energy barrier of hydrogen diffusion in the interior of superfine LPSO structures. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.