Crystallinity of ZnO thin film for film bulk acoustic wave (BAW) resonator has been successfully improved by using Pt/Al bottom electrode. The lattice mismatch of ZnO c-plane and Pt (111) plane is as small as 1.4% so that we could obtain good crystallinity ZnO film on Pt surface. The effective electromechanical coupling coefficient (kt e f f 2 ) of the BAW resonator that we fabricated with Al/ZnO/Pt/Al/SiO 2 structure is 5.6%. It is larger than that of Al/ZnO/Al/SiO 2 structure BAW resonator, whose value is 4.9%. The kt e f f 2 is improved by using the Pt/Al bottom electrode without decreasing the quality factor. The kt e f f 2 of BAW resonator strongly depends on the crystallinity of ZnO film. Using Pt buffer layer to protect Al electrode from its oxidation is very effective to improve the crystallinity of ZnO film and the kt e f f 2 of BAW resonator.