In this study, the effects of a long post-deposition thermal annealing on the surface acoustic wave (SAW) properties of polycrystalline (poly) aluminum-nitride (AIN) thin films grown on a 3C-SiC/SiO2lSi structure were investigated. For the thin film annealed at 600°C for 4 h, the degree of (0002) orientation of the thin film improved due to the decrease in the full width at half maximum of the (0002) AIN rocking curve (0.028°). This led to an improvement in the electromechanical coupling coefficient of the SAW structure (0.019 percent). The average crystallite size of the thin film, annealed at 1000°C for 1 h, was smaller than that of the as-deposited film, which led to a substantial improvement in the insertion loss of these SAW structures (8.79 dB). The residual stresses in the films were hardly affected by the thermal annealing processes.