This paper reports on the air-gap type thin film bulk acoustic wave resonator(FBAR) using ultra thin wafer with thickness of 50$mu extrm{m}$. It was fabricated to realize a small size devices and integrated objects using MEMS technology for flexible microsystems. To reduce a error of experiment, MATLAB simulation was executed using material characteristic coefficient. Fabricated thin FBAR consisted of piezoelectric film sandwiched between metal electrodes. Used piezoelectric film was the aluminum nitride(AlN) and electrode was the molybdenum(Mo). Thin wafer was fabricated by wet etching and dry etching, and then handling wafer was used to prevent damage of FBAR. The series resonance frequency and the parallel frequency measured were 2.447㎓ and 2.487㎓, respectively. Active area is 100${ imes}$100$mu extrm{m}$$^2$.Q-factor was 996.68 and K$^2$$\_$eff/ was 3.91％.