This paper presents a two-dimensional FEM (Finite Element Method) modeling and simulation of a surface acoustic wave (SAW) resonator based on a layered Pt/AlN/Sapphire structure. Such structure that exploits the electromechanical coupling of piezoelectric film is of high interest for harsh environments. By harsh environment we mean any environment that could hinder the operation of the device. Hardness can come from a variety of sources, and examples include the following: High pressure, High temperature, Shock/high vibration, Radiation, Harsh chemicals, etc. As part of this work, we are looking for high temperature sensor applications and only operating drifts due to temperature will be studied. SAW resonator is made from piezoelectric thin film Aluminum Nitride (AlN) layer on Sapphire substrate. Modal analysis is used to determine the eigen mode and the eigenfrequency of the system and the study of the frequency domain is used to determine the response of the model under influence of a harmonic excitation for one or more frequencies. In the FEM modeling, various parameters of the surface waves in the films, such as the surface velocity, the displacement of the piezoelectric thin film, the electrical potential, the electromechanical coefficient (k2), and the quality factor (Q) were studied. A comparative study between modeled and experimental curves showed a good agreement and allowed to validate our simulation method. Finally, a FEM study of the influence of normalized thickness of AlN thin film on resonator performances was carried out and compared with theorical results of literature.