The crystal structure changes in the hydrogenation and dehydrogenation process have been investigated by the in-situ synchrotron X-ray diffraction measurement to elucidate the mechanism of hysteresis phenomena in equilibrium hydrogen pressure. The hysteresis phenomena were observed on the crystal lattice. The lattice constants and volume of alpha-phase (hydrogen solid solution) and beta-phase (hydride) in the dehydrogenation process with lower equilibrium pressure were smaller than those in the hydrogenation process with higher equilibrium pressure. This phenomenon is opposite to the general relationship between the lattice volume and the equilibrium pressure. The hysteresis phenomenon was also observed in the diffraction peak widths. The peak widths of beta-phase in the dehydrogenation process were wider than those in the hydrogenation process. The hysteresis phenomena may be related to lattice defects and the hydrogen content in crystalline regions. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.