Abstract: Thin film bulk acoustic resonators(FBARs) are feasible to detect gamma irradiation by experiments, but the sensing mechanism has not been thoroughly studied. According to two different FBAR structures, two sensing mechanisms are proposed to explain the resonance frequency shift under gamma irradiation. One FBAR structure is four-layer stacked(metal layer-piezoelectric layer-oxide layer-metal layer), after gamma irradiation, it will form a voltage in the radiation sensitive layer(the oxide layer), which is equivalent to impose a DC voltage on the piezoelectric layer, hence the resonant frequency will shift. There is a semiconductor layer between the oxide layer and the piezoelectric layer in the other FBAR structure, and a voltage formed in the oxide layer after irradiation will change the surface potential of the semiconductor and then change the capacitance of the semiconductor space charge layer and finally change the resonant frequency. The results of two different mechanisms are obtained by simulation and compared with the results of the related literature. It is found that the trend of frequency shift and the magnitude of frequency shift are the same, which suggests that the two mechanisms are feasible.