This paper presents an air gap eccentricity fault Diagnosis of a salient Pole three phase synchronous generator. The detection and diagnosis of this type of fault is based on analyzing armature and field current signatures, using FFT . Therefore, the synchronous generator, under this type of fault, is modeled by using finite element method (FEM). Also practical results are performed in the lab with deliberately air gap eccentricity faults, by using samples of bearings malfunctioning or the removal of single or pair pole. The modeling and experimental results show that FEM is a reliable approach that can be used to model synchronous generator under electrical and mechanical faults