Four exothermic reactions of (1) hydrogen absorption forming crystalline (c-) and amorphous (a-) DyNi2Hx, (2) the transformation of c-DyNi2Hx to a-DyNi2Hx, i.e., hydrogen-induced amorphization (HIA), (3) the precipitation of DyNi5 in a-DyNi2Hx and (4) the decomposition of the remaining amorphous hydride into DyNi5 and BiF3-type DyH3 were observed sequentially when the C15 Laves phase DyNi2 was heated using a pressure differential scanning calorimeter (PDSC) at 2.0 MPa H2 or higher hydrogen pressures. On the other hand, (1) the formation of c- and a-DyNi2Hx, (2) partial HIA and (3) the decomposition of the mixture of a- and c-DyNi2Hx into DyNi5 and DyH3 (or DyH2) were observed below 2.0 MPa H2. Simultaneous formations of c- and a-DyNi2Hx at the first exothermic peak were explained by the local temperature increment resulting from the low activation energy for hydrogen absorption forming c-DyNi2Hx. On the other hand, the formation of DyNi5 during the decomposition was explainable in consideration of the large formation enthalpy of DyNi2. Furthermore, the mechanism of the phase transformations of DyNi2 heated in a hydrogen atmosphere was discussed on the basis of the experimental results such as the pressure dependence of Tp and Tp/Tm (the peak temperature/the melting point of DyNi2), the activation energy EA, and the enthalpy change ΔH.