Load relaxation tests, creep and abrupt stress change tests, and tension tests were performed on unirradiated, annealed Zircaloy-2 and stress-relieved Zircaloy-4 at temperatures ranging from 473 to 648 K. Experimental results were analyzed by using a unified phenomenological constitutive model originally proposed by Hart. All the parameters included in the model were determined. Using the parameters, the model was employed to integrate numerically along the loading conditions under which experiments were performed. The model showed reasonable accuracy and was demonstrated to be applicable for describing nonelastic deformation of Zircaloy cladding.