Fundamental and overtone modes of quartz resonators with high frequencies have been studied to increase the sensitivity of QCM gas-sensor. Polystyrene film was plasma-polymerized onto the electrode surface of an AT-cut quartz resonator and then used as a molecular recognition membrane to detect gas-phase analytes. In this study, handmade oscillators with superior stability were used to vibrate each mode of AT-cut quartz resonators. Deriving the theoretical mass sensitivity of an AT-cut quartz resonator vibrating at the overtone mode, confirmed the fact that the theoretical value was the same as the result obtained from the experiments.