A nano-/ultrafine grain gradient microstructure, which is composed of high-angle grain boundaries (HAGBs) and low-angle grain boundaries or subgrains of dislocation-twin, was fabricated in Zircaloy-4 using surface mechanical rolling treatment (SMRT). Thermal stability of gradient microstructure has been investigated through characterizing the evolution of microstructure during post-SMRT annealing treatment from 400 to 600 degrees C using optical microscopy and transmission electron microscopy. Experimental results show that the gradient microstructure exhibits a good thermal stability at 400 degrees C, since the overall grain size remains similar, except a decrease in dislocation density due to recovery. In comparison, a hierarchical microstructure is formed after annealing at 600 degrees C. An obvious grain growth was observed at the depth of 50 mu m. The activation energy for grain growth of nanograined Zircaloy-4 is estimated to be ~ 161 kJ/mol between 400 and 600 degrees C. Nano-/ultrafine grains predominantly consisting of HAGBs have the highest thermal stability. Both yield strength and ultimate tensile strength of Zircaloy-4 decrease due to anneal, specifically at 600 degrees C.