An X-ray microbeam study and a polycrystal finite element model of a section of a thick polycrystalline aluminum film on a silicon substrate are used to investigate the effect of microstructure on thermal stress variability. In the X-ray microbeam study, the grain orientations and deviatoric elastic strain field are measured at the subgrain level in the film during and after two thermal cycles. A finite element model of the observed grain structure is created and modeled with an elastoviscoplastic crystal constitutive model that incorporates film thickness and grain size effects as well as dislocation entanglement hardening. The experimental and simulation results are compared at both the film and subgrain scales. While the experiment and model agree fairly well at the film level, the experimental results show much greater elastic strain variability than the simulations. In considering the grain size effect, the experiment and model both predict a similar Hall–Petch coefficient, which is consistent with literature data on free standing aluminum thin films.