The transmitting beams are usually approximated as plane wave or spherical wave in free space optical communication (FSO) systems. But the actual beam is a Gaussian beam, and there are many differences when we study the performance of FSO system with the approximation model. By using the methods of theoretical analysis and simulation, without considering the impact of the inner and outer scales of turbulence, we analyze the difference of aperture averaging effect among different transmitting beams. The differences of bit-error rate (BER) performance are then analyzed. The results show that Gaussian wave has apparent impact on the improvement of BER performance by aperture averaging, and saturation is reached firstly, followed by spherical wave. Plane wave is relatively gentle.