For the ammonia (NH3) and lithium hydride (LiH) system, which is one of the promising hydrogen storage materials, the correlation between particle size of LiH and reactivity with NH3 is investigated to understand the kinetic properties of the system. The pristine LiH shows low reaction yield of less than 10% due to the slow kinetics even after the reaction for 24 h. The activation by the ball-milling drastically improved both of reaction rate and yield, and then more than 80% of reaction proceeded for 24 h in the case of the LiH with the smallest particle size. Thus, the particle size strongly affects the kinetic properties. Furthermore, a thickness of possible NH3 diffusion into the LiH surface for the 24 h reaction is estimated to be 20 nm assuming that the LiH particles are an ideal spherical shape. Therefore, the rate-determining step of the system would be the NH3 diffusion process. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.