A full-field, noncontacting interference measurement for roughness of laser mirrors has been developed. Suppose the probability density function of the surface height distribution is Gaussian. The relationship between the interference fringes contrast and the ronghness of a mirror is deduced. Two laser mirrors with different roughnesses were measured. The data measured are in agreement with the results measured with a 6JA Microscopic Interference Profilometer. The range of measurement is 0.01 to 0.12 μm and the accuracy of measurement is up to 0.002 μm theoretically. Practically, this work may be useful for controlling the process of mirror polishing.