A new method is introduced for the evaluation of experimental stress–strain dependence in thermally cycled thin films. The method is demonstrated on the analysis of an Al thin film on a Si(100) substrate characterized using in situ high-temperature X-ray diffraction 25–450 °C. Diffraction data are used to evaluate in-plane elastic strain in the film as a function of thermal strain originating from the mismatch of thermal expansion coefficients (TECs) between the film and the substrate. The magnitude of the thermal strain is calculated from experimental TECs of the film and the substrate at every measurement temperature. By relating in-plane stresses to thermal strains, an experimental stress–strain dependence for the Al thin film is obtained. The proposed method allows one to identify elastic behavior and to quantify plastic strain in the film. Finally, advantages of the method are discussed in particular its independence from using TECs reported in the literature.