Abstract: The transmission line method is used to deduce the input impedance equation of thin film bulk acoustic resonator (FBAR) in this paper.The different combinations of FBAR are studied on by the equation, and the material and the thickness of electrode effects on FBAR are obtained, which includes the maximum effective electromechanical coupling factor theory.The results show that the effective electromechanical coupling factor verifies with the electrode and the density of the electrodes, and the electromechanical coupling factor descends with the thickness of electrodes, which is obvious if the density of electrode is small.On the other hand, the effective electromechanical coupling factor ascends with the density of electrodes, which is obvious if the thickness is large.These results can be applied in the design and the optimization of FBARs.