Abstract: The mechanical tests and EBSD (electron backscattered diffraction) analysis of industrially pure copper foil with thickness of 25~300 μm rolled by asymmetric rolling process had been conducted to study the size effect phenomenon. The size effect of copper foils with different thickness was found， i.e. the tensile strength and hardness first increased and then decreased with the decrease of foil thickness when the thickness reached the critical value. The mechanical property trended to change at the thickness threshold. The work hardening with the decrease of thickness only happened when the foil thickness was bigger than the thickness threshold. And when the foil thickness was less than the thickness threshold， work softening happened. The reason why size effect happened was that the grains were stretched， flattened even pulled off as the thickness of foil decreased so little new grains provided vast grain boundary which made the dislocation density decrease.

Key words: foil, asymmetric rolling, size effect, work softening, thickness threshold