In this paper, the relationship between characteristic of surface roughness and average contrast of laser speckle pattern has been derived directly from the Fresenel-Kirchhoff diffraction formula by means of light intensity distribution. Two forms of autocorrelation function have been considered, and the corresponding results are very similar. The method of measuring surface roughness using average contrast of laser speckle pattern can be used in measuring roughness of precise surfaces with high accuracy and high resolving power.