The thermal stability and hardness behavior of ultrafine--grained (UFG) copper produced by equal channel angular pressing (ECAP) under the condition of annealing were studied by differential scanning calorimeter (DSC) and micro--hardness tests, and the microstructural changes of this material under uniaxial compression or cyclic deformation at temperatures ranging from room temperature to 300 ℃ were examined by electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and by transmission electron microscopy (TEM). It was found that under annealing even at a certain temperature below recrystallization temperature, UFG copper would exhibit a structural evolution, i.e., recrystallization and grain coarsening, the process of which may happen gradually at a low developing rate, so that the DSC response curve as a functional of annealing time cannot detect such a process. The grain coarsening behavior of UFG copper under high--temperature compression is related to the strain rate, i.e., the higher the strain rate, more remarkable the localization of grain coarsening becomes; the lower the strain rate, many more grains become coarsened integrally. Comparatively speaking, the grain coarsening induced by high--temperature cyclic deformation takes place more notably and uniformly, and some typical dislocation arrangements, like dislocation walls and dislocation cells etc., can be observed in some coarsened grains. Also, the inhomogeneity of grain coarsening under high--temperature deformation was quantitatively discussed in terms of a ratio V of maximum grain size (D_max) to average grain size (D_aver), which is available for the coarsened grains.