Catalytic properties of nickel (Ni) nanoparticles for thermal ammonia decomposition were investigated. The nanoparticles were synthesized from Ni(C5H5)(2) in/on pores of zeolite with an aim to prevent diffusion and sintering of the nanoparticles at high temperature. The Ni nanoparticles were smaller than 5 nm and maintained their nano-size after the NH3 decomposition reaction at 500 degrees C, whereas Ni particles synthesized by a conventional impregnation method formed large particles, such as 50 nm, after the reaction. The Ni nanoparticles showed much higher activity than Ni particles synthesized by the conventional impregnation method. By the investigation of kinetic properties, it was confirmed that the frequency factor was related to the high catalytic activity. Therefore, both high dispersion level and high thermal stability brought Ni nanoparticles the enhancement of their catalysis. (C) 2014 Elsevier B.V. All rights reserved.