Sm3H7 nanoparticles have been successfully produced by hydrogen plasma-metal reaction (HPMR) method, and then used to synthesize ammonia at 298 K and 1 atm. The morphologies of the Sm3H7 nanoparticles before and after reaction were investigated by transmission electron microscopy, and the crystal structures at different steps by X-ray diffraction. Nessler's test was adopted to detect ammonia. It was found that the passivated Sm3H7 nanoparticles possess polyhedron shape and smooth surface, with the average size of about 50 nm and the specific surface area 11.2 m(2) g(-1). It was proposed that Sm3H7 nanoparticles react with oxygen and nitrogen to form ammonia, but ammonia production is not observable in the case of coarse particles. After ammonia synthesis, the morphology Of Sm3H7 nanoparticles changes into spongy surface and the mean particle size and specific surface area increase to 100 nm and 28.6 m(2) g(-1), respectively, due to the release of hydrogen. The hydrogen conversion percentage from samarium hydride is estimated to be 1.5%. Without O-2, Sm3H7 nanoparticles cannot react with N-2 or N-2 + H-2 at 298 K and 1 atm. (c) 2005 Published by Elsevier B.V.