To study the behavior of hydrogen corrosion at the surface of U, U–2.5wt%Nb alloy and U–5.7wt%Nb, a gas–solid reaction system with an in situ microscope was designed. The nucleation and growth of the hydride of the alloy were continuously observed and recorded by a computer. The different characteristics of the hydrides on U metal and U–2.5wt%Nb showed that the later alloy is more susceptible to hydrogen corrosion than the former. The growth rate of hydride of U–2.5wt%Nb, calculated by measuring the perimeter of the hydride spots recorded by the in situ microscope, exhibited a reaction temperature dependency in the range of 40–160°C, for pressure of 0.8×10 5 Pa. An Arrhenius plot for growth rate versus temperature yielded activation energy of 24.34kJ/mol for the hydriding of U–2.5wt%Nb alloy. The maximum hydriding rate was obtained at 125°C, whose thermodynamics reason was discussed.