This study uses Computational Fluid Dynamics (CFD) to investigate the effect of guide vanes on power output of turbine installed in a solar chimney. A full scale three-dimensional CFD simulation of solar chimney power plant based on Manzanares prototype is performed to simulate the air flow inside the solar chimney. First, the flow field is simulated in presence of guide vanes with no turbine inside the chimney. Three guide vanes with different tip angles of 30°, 60° and 90° are considered. It is observed that the presence of guide vanes significantly affects the flow direction and the heat stored inside the blade chamber. Moreover, the change in output power of solar chimney under different turbine rotational velocities and different guide vanes is studied. A twelve-blade turbine is placed right above the exit of the guide vanes at the start of chimney. Various cases including three angles, 30°, 60°, and 90° guide vanes, and three rotational velocities, 40, 80, and 120 rpm are run to find output power. The Multiple Reference Frame (MRF) method is used to treat the rotation of the turbine. The analysis of power output of the turbine shows that the maximum power of 82.2 kW is reached by the case of 30° guide vane at 80 rpm. Comparing to the power of the 90° guide vane at the same rotational velocity, which is 66.3 kW, the increase is about 23.9%.