Abstarct: Power system stabilizer (PSS) is used as a control system for improving low frequency oscillation damping. Its use will be effective when it is designed by an appropriate parameters. According to studies, the phase compensation method is generally used for designing power system stabilizers. In this method, the phase angle compensation is obtained baxsed on network, generator and excitation system data. Uncertainty in each of these cases lead to incorrect calculation of the compensation angle and so influence on the design Procedure and will create a significant error in results. Although the data of the network and generators are available in large and complex networks, Uncertainty in model parameters and excitation system is inevitable. Studies to identify the exact model and system parameters in order to eliminate the uncertainty would be time consuming and expensive. In this thesis the effect of this uncertainty on efficiency of power system stabilizer design has been studied. In this case, the uncertainty will effect directly on PSS tuning. On the other hand, the angle compensation will be obtained at a certain mode frequency. Since the excitation system is one of the factors that affect the small signal stability, can be affected on the results of the eigenvalue analysis stability. So if the uncertainty can change the frequency electromechanical mode, may result in the compensation angle and setting of PSS also take effect. Therefore we have to be informed the effect of excitation system models and parameters on the electromechanical mode. The results of the thesis are provided in two chapters: the influence of different exciter models and parameters on electromechanical mode and phase shift compensation.