The mechanical properties of thin films, and thin film-substrate mechanical interaction and adhesion is important for the design of interconnects with good thermal and mechanical stability. As a result, there have been many studies on the residual stress and properties of polyimide (PI) thin films [1-3], metal thin films [4-8], and on the chemical interactions between PI and metal thin films [9, 10]. In addition, several tests have been developed to study the metal-PI adhesion [11]. However, to date there has been no in situ measurement of the actual strain transfer through an intermediate PI layer. In this study an in situ X-ray tensile test was applied to three-layer structures of Ni-PI thin film-Cu thin film to measure, for the first time, the strain transfer through a PI layer. The geometry of the samples is shown in Fig. 1. The experimental procedure was as follows. Nickel dogbone samples with gauge length dimensions of 29mm × 6.4 m m x 0.8 mm were polished to a 1/tm mirror finish. PMDA-ODA (5878) was spun on at 3000 r.p.m, for 30 s, resulting in 6 #m PI. After curing the PI at a maximum temperature of 380 °C, 4 #m Cu was evaporated on to the PI. The mechanical test involved loading the substrate with a mini-tensile tester equipped with a load cell. The applied strain was monitored with a resistive strain gauge on the back of the sample. At each increment of applied strain the X-ray stress was calculated for both the Ni and the Cu layers from the change in the lattice spacings. The X-ray stress was determined by using a standard 6 tilt d versus sin2~p analysis on the Cu 331 and Ni 420 peaks. The d versus sin2~p behaviour was linear, there was no texture in the metals and the X-ray elastic constants were calculated theoretically, and verified experimentally. The details of X-ray stress analysis are reviewed in [6]. Consider the stress-strain transfer geometry in a multilayer thin film structure (Fig. 2). Stress/strain is