Analytical and finite-element models of a localized-mass sensor fabricated with thin-film bulk acoustic wave resonator (FBAR) are reported. While our group demonstrated FBAR-based localized-mass sensors, no previous modeling of these sensors is found in the literature. The finite-element model (FEM) defines the boundary conditions and performs parametric analysis of the sensor's mass loading, whereas the analytical approach takes advantage on a modified Mason's model to describe the transmission-line circuit of the sensor. The sensitivity of the resonance frequency to location and size of mass deposition has been studied. Both the experimental and modeled responsivities exhibit a nonlinear dependence on the location and size of the localized-mass.