Technological advances constantly set new challenges for materials development. The miniaturisation of electronic devices demands the migration of metallurgy from macro/micro to the nanoscale, thus requiring a re-definition of existing and classical concepts in this field. The present study reports on the behaviour of pure Cu nanowires with diameters ranging from 40 to 140 nm heated in a low-pressure environment within a transmission electron microscope. The response of Cu nanowires was investigated at different temperatures up to 1123 K and analysed using electron-microscopy techniques, revealing both volumetric and shape changes over time. Sublimation, with a steady-state length reduction of the nanowires, was identified as the dominant effect of such heating. Additionally, it was detected that sublimation occurred not only at temperatures above ≈ 1023 K, where Cu has a higher vapour pressure than the column pressure of the electron-microscope, but also at temperatures as low as 923 K. This behaviour is explained by the presence of active regions at sharply curved regions at the nanowire tip and the imbalance of evaporation and redeposition rates of Cu atoms due to the experimentally-induced loss of vapor atoms. The study of Cu nanowires at the nanoscale with the electron microscope facilitates the elucidation of some fundamental aspects of the emerging science of nanometallurgy.