We consider the effect of surface steps on RHEED intensities. A multiple scattering theory of RHEED described by Maksym and Beeby is used to calculate intensities for RHEED by the Al(001) surface and the effect of steps is studied by considering periodic arrays of monolayer steps. The steps are found to cause two characteristic changes in the RHEED intensities. One is that the intensity of the specular spot is diminished when the incident beam azimuth is parallel to the step edges. The other is that when the incident beam azimuth is perpendicular to the step edges the heights of the peaks in the RHEED intensity curves depends systematically on the number of atoms in the topmost layer. The latter effect is consistent with the diffraction intensity oscillations recently reported in studies of molecular beam epitaxial film growth.