The intermetallic compound ZrNi was mechanically ground under various partial pressures of hydrogen. At the beginning of the grinding, hydride phases (ZrNiH and/or ZrNiH3) are formed, which lowers the free energy of the compound, and the particle size are effectively reduced. Moreover, with progression of the grinding, an amorphous phase starts to grow. Finally, depending on the partial pressure of hydrogen, a composite particle of the amorphous and crystalline phases is formed. Consequently, such a grinding technique under various partial pressures of hydrogen promotes the synthesis of fine composite particles, each of which consists of amorphous hydride and the crystalline hydride phases. In addition, the grinding enhances the decomposition to form ZrH2 even at room temperature. During the heating process under an argon atmosphere, the dehydrogenation of ZrH2 can progress even at low temperatures of around 750 K, which is caused by preferential crystallization of the amorphous phases in the fine composite particles.