An experiment on laser speckle diffraction and measurement of speckle intensity has been conducted. The results show that the Fraunhofer diffraction pattern or quasi-Fraunhofer diffraction pattern of the measurement aperture may be observed, when the raito of the average speckle diameter to the aperture diameter is equal to or larger than one, dr/d0>1. When dr/d0>>1, the negative exponential probability density function of speckle may be obtained, and the speckle contrast may be used to characterize the scattering surface roughness.