The rate-limiting step in hafnium-hydrogen reaction was investigated by comparing experimental kinetic data with dependence of overall reaction rates on temperature and pressure for each step of the reaction. Kinetics of hafnium hydride formation using powder-type specimens was studied over a wide range of temperature(405$\sim$1100K) under hydrogen pressure of 580 torr. The activation energy of 10.11 kcal/mole for hydrogenation reaction was obtained from the slope of the linearly decreasing region of In $U_s$ ($U_s$ : reaction rate) versus 1/T curves. In addition, the behavior of hydrogen during thermal analysis of hafnium hydride formed at 1100K under the hydrogen pressure of 580 torr, was investigated by use of a gas chromatograph. Activation energies, 8.53 kcal/mole and 11.28 kcal/mode, for hydrogen evolution from $\delta$-phase hafnium hydride were obtained by measuring the change of peak temperature of the hydrogen evolution curves at different heating rates. X-ray analyses showed that $\delta$-phase hafnium hydride transforms to $\delta$-hafnium as hydrogen was evolved from the $\delta$-phase hafnium hydride. Thermal analyses and X-ray diffraction analyses showed that the first peak of the two evolution peaks for the $\delta$-phase hafnium hydride is attributed to evolution of dissolved hydrogen in the $\alpha$-phase hafnium while the second peak to phase transformation of the $\delta$-phase hafnium hydride to the $\alpha$-phase hafnium. Hence it was concluded that the rate-limiting step in hydrogenation reaction of hafnium powder is the formation of hafnium hydride at the hydridemetal interface.