This paper studied the resonance properties of Film Bulk Acoustic Resonator (FBAR) with initial stress in thickness shear mode. Based on the fundamental piezoelastic equations of piezoelectric material with initial stresses, the vibration equation of ideal FBAR under sine excitation voltage is derived. The solution is then obtained by using the boundary conditions. The series and parallel resonance frequency are listed, and the effects of initial stresses on resonance frequency, bandwidth, and effective electromechanical coupling coefficient are discussed. A numerical example is presented to show the effects. These results are very useful for the improvement of FBAR performance.