100 nm thick Cu films were prepared on the elastic polyimide substrates and a yield stress of the ultra-thin Cu films was measured by means of high elasticity of the polyimide substrate. The behaviors of deformation and fracture of the Cu films were also investigated. The results show that the yield stress of the 100 nm thick Cu films can be measured by Cu/polyimide composites. The yield stress of the ultra-thin Cu films is significantly higher than that of the thicker Cu films. Failure of the 100 nm thick Cu film under tensile load is characterized as a model and intergranular fracture. It is suggested that the higher yield stress of the present 100 nm thick Cu films is owing to strong constraints of nanoscaled film thickness and grain size on dislocation motion in small dimensions.