In this study we present investigation on the anelastic behavior of sputtered 1 [.proportional]m thin Cu films. Most of the literature that reports on the mechanical properties of thin metallic films is based on substrate curvature measurements. We have developed a new version of a bulge tester that combines the capacitive measurement of the bulge deflection of a membrane with a resonance frequency measurement of the residual stress in the membrane. A Cu membrane is plastically deformed to a pre-determined strain by controlled gas-pressure bulging of the membrane. After the bulging stress is removed, the residual tensile stress, which has been decreased by the plastic deformation, is then determined by measuring the resonant frequency as a function of time. Immediately after plastic straining, the residual (tensile) stress of membranes was observed to increase. At room temperature a maximum stress was typically reached in the order of an hour. At still longer times the stress decreased again as a result of creep. The transient increase in stress following plastic straining grew larger as the amount of plastic strain produced by bulging was increased. With higher temperatures the transient became both faster and larger. A model is presented that based on the mechanism of thermally activated glide separates the microstructure in a class of “soft” and “hard” grains solving the issue of an “apparent” increase in strain energy as a function of time after deformation.