The well-established enhanced mechanical performance of ultrafine-grained (UFG) materials is often accompanied by poor thermal stability which precludes their use in some applications. To provide fundamental insight into the problem of thermal stability, we studied the microstructure of two different UFG microstructures in Cu: one with lamellar grains and low-angle grain boundaries (GBs) and another with equiaxed grains and high-angle GBs. These distinct microstructures were obtained using equal channel angular pressing (ECAP) for 2 and 16 passes, respectively. Our results show that both microstructures exhibited a similar yield strength, but the material processed using two passes (ECAP-2) exhibited a higher thermal stability when compared to that of ECAP-16. Thermodynamic calculations indicate that the ECAP-2 Cu has a lower stored energy (0.43 J/g) relative to that of the ECAP-16 Cu (0.66 J/g) because the former microstructure has lower GB energy and lower GB volume fraction relative to the latter one. Kinetic analysis revealed that the ECAP-2 Cu has higher activation energy of recrystallization (94 kJ/mol) as compared to that corresponding to the ECAP-16 Cu (72 kJ/mol).