Abstract: The phenomena of loss in the piezoelectric materials is analyzed, and parameters of lossy piezoelectric resonator can be characterized by complex parameters. The equivalent circuits of lossy resonators can adopt in two forms: one way is changing the real circuit parameters into complex ones, while the equivalent circuits hold unchanged, the other is adding the dissipation component in the equivalent circuits of the corresponding lossless resonators. Based the impedance （or admittance） equation, the connection of the resonance （or anti-resonance）frequency, the frequency displacement with the magnitude of loss and piezoelectric coupling factor are analyzed in detail.