A Cu/polyimide thin film couple prepared on a thin quartz reed has been used to study interfacial stress relaxation during thermal cycling between room temperature and 400 °C. The polyimide thickness varies from 0 (no polyimide at all) to 10.5μm while the Cu thickness was fixed at 0.53μm. The average copper film stress has been calculated from the curvature of the quartz reed. The information clarifies the relation between the polyimide thickness and the average Cu film stress. The Cu/polyimide interfacial morphology after thermal cycling has also been examined using the cross sectional TEM technique. The results suggest that the interfacial stress is partially released through the deformation of polyimide near the Cu/polyimide interface.