Abstract: A High-overtone Bulk Acoustic Resonator (HBAR) consisting of a piezoelectric film with two electrodes on a substrate exhibits high quality factor (Q) and multi-mode resonance spectrum.The resonance spectrum of the HBAR is given,then the resonance spectrum characteristics of mechanical quality factor (Q_M) are analyzed by taking each layer's structure (thickness) and material properties (characteristic impedance and mechanical attenuation factor) into account.The relationship between Q_M and the thickness of the substrate or piezoelectric film is shown by a series of curves corresponding to different modes.Q_M at a certain frequency increases with the oscillatory behavior as the substrate thickness increases,and eventually tends to the mechanical quality factor of the substrate,while it decreases in a wave-like changes as the piezoelectric film thickness increases.Q_M decreases as the frequency or number of mode increases for a given HBAR.In addition,it has little effect on the variation rules as considering the thickness of the electrode.In order to obtain larger Q_M,the structure of Al/AIN/Al/Sapphire or YAG with appropriate thickness of the substrate,piezoelectric film and electrode should be selected.