This paper reports post-annealing of zinc oxide (ZnO) films on flexible foil substrates in order to improve the functional and acoustic wave sensing performance. ZnO films of 5 m thick were deposited onto aluminum foils (50 μm thick) using magnetron sputtering and then annealed in air at different temperatures between 300 and 500oC. Effects of post-annealing on structural, optical and device properties of the ZnO films and ZnO/Al foil acoustic wave devices were investigated. A temperature of 350oC was identified as the optimized annealing temperature, which resulted in good light transmission, improved crystallinity, reduced film stress/defects, and increased amplitude of reflection signals of both Lamb and Rayleigh waves. The annealed ZnO/Al acoustic wave devices demonstrated a large temperature coefficient of frequency and a good linearity, revealing the potential for precision temperature sensing.