Abstract: QCM is a precision mass detector for measuring the gas phase thin-film deposition. Because the correlation between frequency and mass can be affected by many uncertain factors, the sensitivity of QCM often plays an important role in securing accurate results. In this paper, from the QCM theory and the measurement of the thin-film thickness, the linear relation and the method for determination of the mass sensitivity of the gas phase thin-film QCM are discussed, where the linear relation is the key. The feasibility of the method based on the thickness measurement of the thin-film deposited on the sensor is discussed.