Abstract Thermal stability of ultrafine grained (UFG) structure of 99.9% pure copper produced by eight equal channel angular pressing (ECAP) passes was studied. The annealing experiments were conducted at temperature range from 180 °C to 300 °C in a tube furnace under argon as a protecting gas. The dwell times were in the range from 10 min to 300 min. The electron backscattering diffraction (EBSD) analyses were performed before and after annealing at exactly the same area in order to quantify the changes of UFG structure. More advanced analysis of the EBSD results based on kernel average misorientation (KAM) parameter was performed.