The need for clean sustainable resources of energy have increased utilization of renewable energies such as solar resources. With increase of penetration of photovoltaic (PV) power plants, more conventional power plants may be replaced by PVs. PV power plants have some different properties with respect to synchronous generators such as using fast controllers and lack of a rotating mass. Due to these problems, PV power plants added to system are expected to decrease whole power system inertia and cause changes in power system operational and dynamic characteristics. Therefore, investigating the effects of adding PV power plants on power systems is necessary. The main purpose of thi thesi is to investigate the effect of high penetration of PV generations on power system stability. To achieve this aim, the three-phase single-stage dynamic model of PV system are added to power system dynamic model. Nowadays, the three-phase single-stage model is commonly used for grid connected PV systems. By some of the modificatio and extensions proposed in the testis, the model can also be used for very large scale PV power plants within range of hundreds of megawatts. The PV model has been integrated with IEEE 39-bu system to study effects of PV power plant on both steady state and dynamic characteristics including rotor angle ,frequency ,and voltage stability. Additionally, some other key factors such a installing place and voltage control strategies have bee investigated. Two studies ,small signal and transient rotor angle stability are individually carried out in the section of the analyzing impacts of PV generation on the rotor angle stability. In the small signal rotor angle stability, the damping of system electromechanical modes of linearized system equations is investigated for different cases. The transient stability is studied through analyzing amplitude of rotor angle oscillations of system generators under different large disturbances. Frequency stability is studied with tripping a generator at different penetration levels. Finally, voltage stability is studied considering to short-term voltage stability and system load ability. Short-term voltage stability is analyzed through the time-domain voltage curves of some buses. Load ability is also assessed through P-V curves at different PV penetration. Keywords:Photovoltaic power plants, renewable penetration levels, steady state studies, dynamic studies