The process of solid gas reaction milling in hydrogen was applied to achieve full disproportionation and microstructure nanocrystallization of NdFeB alloy. The NdFeB alloy was disproportionated by room-temperature reactive ball milling in hydrogen, with the mechanical energy serving as the driving force. The reaction progress during milling was examined by hydrogen absorption measurement, and the corresponding microstructure change was characterized by X-ray diffraction (XRD). The mechanism of mechanically activated hydrogenation disproportionation reaction of NdFeB alloy was investigated. The disproportionation kinetics of an as-cast Nd16Fe76B8 (atomic ratio) alloy during reaction milling was experimentally studied by in-situ monitoring of the hydrogen absorbed by the alloy in the vial. On the assumption that the impact energy to activate the reaction plays a role analogous to that of the thermal energy for the conventional thermally activated process, a theoretical model for the kinetics of the mechanically assisted disproportionation was proposed. By fitting the experimental data with the theoretical model, the kinetic equation and the energy utilization efficiency under different milling conditions were obtained. The effect of milling parameters and hydrogen pressure on the disproportionation kinetics was evaluated and the underlying mechanisms were discussed by referring to the mechanical energy input intensity and the Gibbs free energy change for the reaction. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.