A kinetic Monte Carlo simulation of polycrystalline thin film growth has been developed that enables the effects of stress to be included. The stress is modeled in terms of processes that happen at the grain boundary, i.e., the formation of new grain boundary lengths and diffusion of atoms from the surface. In this work, the simulation has been used to study the effect of grain size on the thin film stress and surface morphology evolution. Results are presented for different grain sizes, temperatures, and growth rates and the kinetics of relaxation when the growth is terminated. The results are interpreted in terms of rate equations developed to explain thin film stress evolution. © 2020 Author(s).