The effects of thermal annealing of W/SiO2 multilayer Bragg reflectors on the resonance characteristics of ZnO-based film bulk acoustic resonator (FBAR) devices are presented for the first time. The resonance characteristics could be significantly improved due to thermal annealing. FBAR devices with Bragg reflectors annealed at 400degreesC/30 min show excellent resonance characteristics in terms of return loss and Q-factor.