In free-space optical communication, the impact of aperture averaging of the receiver on the bit-error rate is studied. The log-irradiance variance is a parameter that reflects the log-intensity fluctuation of a certain point on the receiving aperture. If the bit-error rate solely depends on log-irradiance variance, the result is just for the instance of point receiver. The covariance of log-irradiance is a parameter which is fluctuating related to the spacing between two points at the receiver plane. The aperture averaging effect connects the bit-error rate and covariance of log-irradiance. Two kinds of models are considered, spherical wave and plane wave, and the diameter of the receiving aperture is supposed as the first Fresnel zone length. The result shows that the bit-error rate decreases significantly when the aperture averaging effect is taken into account.