The effect of surface irregularities upon the scattering performance of a composite rough surface that consists of scattering elements of structure sizes in quite different scale uniformly superimposed one on the other is described and analyzed. The theory is formulated on the basis of the principles of both geometrical optics and physical optics through the introduction of the surface‐slope distribution with a result clear in physical meaning and simple for analysis. Three factors determine the dominating role played by the component irregularities in wave scattering: surface slope, incident angle, and wavelength. An example showing the angular distribution of wave scattering by a composite rough surface at frequencies of 1 and 100 kHz about 7 oct apart is presented. At low frequency, the backscattering is confined to a narrow region of about 10° centered at the direction of normal incidence, while at high frequency, the angular distribution expands to about 150°. As for the bistatic scattering the angular distribution of the scattered acoustic wave increases from about 25° centered at specular reflection to 180°. This effect of the small‐scale surface irregularities is to broaden the angular distribution of the scattered acoustic wave at high frequency.