To study the effect of nanotwins on thermal stability, a comprehensive characterization study was performed on two types of ultrafine grained (UFG) copper samples, with and without nanotwins. The two samples were sequentially heat-treated at elevated temperatures, and the grain size, grain boundary character, and texture were characterized after each heat treatment. The as-prepared nanotwinned (nt) copper foil had an average columnar grain size of ∼700 nm with a high density of coherent twin boundaries (CTBs) (twin thickness, ∼40 nm), which remained stable up to 300 °C. In contrast, the other UFG sample had few CTBs, and rapid grain growth was observed at 200 °C. The thermal stability of nt copper is discussed with respect to the presence of the low energy nanotwins, triple junctions between the twins and columnar grains, texture and grain growth.