This study describes room temperature ozone sensing with a ZnO based film bulk acoustic resonator (FBAR). The resonant frequency of FBAR decreased upon ozone exposure. For 1400 ppb ozone, the frequency downshift was 131 kHz with a response time of 12 s. The frequency decrease of the FBAR sensor was proposed to be due to the density increase of the ZnO film. Ozone can be adsorbed on the ZnO surface by capturing free electrons from the film, which increases the film density. An analytical model was developed to predict the relationship between resonant frequency and ozone concentration. In agreement with the experiment, a hyperbolic function was obtained. © 2011 The Electrochemical Society.