A new method is applied to investigate the relationship between the yield strength and annealing temperature for a Cu film. By Ion Beam Assisted Magnetron Sputtering (IAMS), Cu film with 2.4 µm thickness was deposited on a strip of super high strength steel 37SiMnCrNiMoV, and the specimens were treated by vacuum-annealing at different temperature. The X-ray tensile test was used to measure the longitudinal and transverse stresses and applied strain for Cu film. Based on the experimental results, the equivalent stress s and the equivalent uniaxial strain t e can be obtained. According to the s- t e relation, the calculated proof stress is acquired. The results indicate that the proof stress of the film decreases with the increasing of annealing temperature. When annealing temperature rises from 150ı to 300ı, the decreasing amplitude of proof stress is the largest. The phenomenon can be explained by the recrystallization and microstructure evolution in Cu film during the annealing treatment.