The C15 Laves phase YNi₂, which becomes amorphous YNi₂H_x by hydrogenation, was mechanically ground under various hydrogen partial pressures up to 1.0 MPa to investigate the effect of the mechanical grinding (MG) on the hydrogen-induced amorphization (HIA) processes. Furthermore, the phase separation processes during dehydriding reaction were also examined. The results obtained are summarized as follows. Under the initial hydrogen pressure of 1.0 MPa, a single phase of amorphous YNi₂H_x is observed by grinding only for 180 min, while such an amorphization can hardly occur even after hydrogenation for 10080 min without grinding. However the thermal stabilities of the amorphous phase and the dissolved hydrogen do not change by MG. On the other hand, when ground under the initial partial hydrogen pressure of 0.2 MPa, YNi₂H_x is separated into two phases. The one is the α-phase, in which the solubility of a hydrogen is larger than that obtained by hydriding YNi₂ without MG under the same condition, and the other is the α′-phase. The solubility of a hydrogen in α-phase gradually decreases with increasing the MG time, while the α′-phase is transformed into the amorphous phase upon further grinding. After grinding it for 1080 min, the α-phase is changed into YNi₅. Excess Y left in the phase transformation will be dissolved into the amorphous phase and it will react with hydrogen to form YH₂. These differences of the amorphization processes depending on the initial hydrogen pressures can be understood by considering the free energy variation in each phase by MG.