Most of the submicron crystalline materials processed by means of equal channel angular extrusion (ECAE) exhibit inherent grain size stabilities up to reasonably high temperatures just as the nanocrystalline materials. But when pure Cu single crystal samples were processed by means of ECAE, grains growth occurs at room temperature in some parts of the samples. The microstructures in these areas are analyzed by the electron backscatter diffraction (EBSP) and electron channeling contrast (ECC) techniques. When the magnification rate is rather low, the Grain Boundary Character Distribution (GBCD) results show that the small-angle and Sigma3 twin boundaries are the two main grain boundary types, the small-angle grain boundaries being a little more frequent than the Sigma3 twin boundaries. When larger magnification rates are chosen, the results are similar, except that the percentage of Sigma3 twin boundaries exceed that of the small-angle grain boundaries. The GBCD results show similar characteristics to those of the recrystallized conventional coarse-grained metallic materials.