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It has been shown that pure thickness shear mode can be excited in thin film bulk acoustic wave resonators (FBARs) using AlN thin films with special c-axis tilt angles (46.1° and 90°). The pure shear mode FBARs may be used as sensors in liquid medium with improved sensitivity than the commonly used thickness shear mode quart crystal resonator. In this paper thickness shear mode FBAR has been investigated for viscosity sensor application. The equation for predicting electric impedance of shear mode FBARs with a liquid layer was derived from the basic piezoelectric constitutive equations. The viscosity sensitivity of AlN FBARS was achieved by calculation of resonant frequency shift due to the liquid loading layer. In the calculation, it is assumed that the FBAR is formed by AlN film with 2μm thickness, 300μm by 300μm electrode area and with mechanical quality factor of 400. Three different liquids (water, acetone and olive oil) were used as the loading layer to evaluate the sensitivity of FBAR viscosity sensors. It was found that the viscosity sensitivities of AlN FBAR with 46.1° and 90° c-axis tilted angle are very close, and do not change much with different liquid materials. The simulation results can be used for design and application of AlN FBARs.