We produced, for the first time, monodisperse NiH(x) nanoparticles with particle diameters of 7.0 nm and investigated their magnetic properties. We also produced monodisperse Ni nanoparticles with nearly the same particle diameters as those of NiH(x) nanoparticles as a comparison. The magnetic properties of NiH(x) nanoparticles were quite different from those of Ni nanoparticles. We observed two compositional phases in NiH(x) nanoparticles, similar to bulk material: one is the nearly pure Ni phase with the blocking temperature (T(B)) of 11 K and the other is the hydride phase. We observed T(B) of 40 K in Ni nanoparticles.