Small angle neutron scattering (SANS) measurements were carried out to investigate the spatial inhomogeneity of deuterium concentration and their size distribution in a nanostructured Mg 2 Ni-D system prepared by the reactive mechanical grinding under a deuterium atmosphere. The scattering profile attributed to the deuterium inhomogeneity was observed, and diameters of the scatterer in the compound mechanically ground for 60 min were deduced to be 48 and 12 nm from Guinier and Porod approximations, respectively. Their values agree well with the near-maximum and average sizes of crystallites of Mg 2 NiH 0 . 3 in the compound, which was previously observed by the high resolution TEM.