We present a study of the stress state of polycrystalline 3C-SiC thin films grown on a-SiO2/Si(100) substrates by Low Pressure Chemical Vapor Deposition using tetramethylsilane (TMS). Substrate curvature and X-ray stress measurements indicate a change from compressive to tensile stress with increasing deposition temperature. The different microstructure of the films with growth temperature and the presence of impurities are at the origin of the observed differences in the stress. We speculate the observed intrinsic stress is composed of a tensile and a compressive component. Within the grain boundary relaxation model we calculate the intrinsic stress variations with temperature due to differences in grain size and density of grain boundaries. The compressive component originates from the presence of oxygen impurities within the film, Stress results obtained from membrane deflection are also briefly presented.