We have fabricated the Ni nanoparticles with a clean surface by the gas evaporation method and clarified the hydrogen storage property by using atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and quartz crystal microbalance (QCM) measurements. The diameter has been estimated as 4.0 ± 1.5 nm by AFM observation. The Ni nanoparticles possess a clean surface, because the surface chemical state has been confirmed as mainly metallic state by XPS. The hydrogen storage property has been investigated by QCM from the vacuum to the atmospheric pressure of the hydrogen at 300 K. The plateau pressure of the Ni nanoparticles and a maximum hydrogen storage capacity are about 7 Torr and H/Ni = 0.56, respectively. And the hydrogen release from the Ni nanoparticles has not been observed by QCM. The hydrogen adsorption/desorption reaction have also been observed as the plateau like pressure by QCM. These results indicate that (i) the plateau pressure of Ni is decreased by the nanosizing of the material, (ii) the large specific surface area of the Ni nanoparticles makes it possible to observe the adsorption/desorption reaction and (iii) the Ni hydride is stable under the vacuum condition. The chemical shift of Ni 2p3/2 has not been observed in the XPS spectrum between before and after exposure to the hydrogen. This result implies that there is no strong chemical bonding between Ni and hydrogen or the hydrogen in the lateral surface has been released. Copyright © 2014 John Wiley & Sons, Ltd.