Recently, charged copper clusters of a few nanometers were shown to be generated spontaneously during the thermal evaporation of copper at 1573 K, and were shown. to be the major flux for the film growth. In order to identify the charging mechanism at low temperatures, the activation energy for positive charging was estimated based on the temperature dependence of the current generated during thermal evaporation. The activation energy was similar to 2.81 eV at 1573 K. This low activation energy could be explained by a Saha-Langmuir equation if the copper clusters are formed and undergo surface ionization on the oxidized tungsten. According to this mechanism, the activation energy for charging decreases with increasing cluster size and with increasing work function of the related surface. Based on this charging mechanism, some puzzling phenomena in the thermal evaporation of metals such as the irreproducibility of the process and the degraded film quality with decreasing evaporation rate could be explained. Published by Elsevier Science B.V.