Property of intensity fluctuation over different receiving aperture for collimated beam propagation in atmosphere is got by experimental method. Using normalized variance of intensity fluctuation and aperture averaging factor the property of intensity fluctuation over different receiving aperture is described and the probability distribution of normalized variance of intensity fluctuation measured with different aperture sizes under different atmosphere conditions is compared. Experimental results show that while turbulent level becomes larger, the receiving area corresponding to aperture averaging effect appears obviously becomes smaller. The probability distribution corresponding different receiving area can be represented by Log-Gauss distribution. Skewness, kurtosis and the feature of Gauss fitted data calculated under experimental conditions are compared. The similarities and differences between the relative intensity fluctuation variance probability distribution of the continuous-wave laser and one of pulsed laser are also compared.