Intensity distribution profiles of reflection high energy electron diffraction (RHEED) along the (00) and (01) reciprocal rods are calculated as a function of the grain width, the height difference between neighbouring grains (intergrain roughness), and the surface roughness of the individual grain (intragrain roughness). The calculations include the refraction and the absorption of electron beams and the shadowing effect by the neighbouring grains. The calculations show that the major intensity moves from the three-dimensional (3D) Bragg diffraction to the specular beam as the grain width increases and that a remarkable change in the intensity profiles occurs when the grain width becomes comparable with the decay length of the electron beam in the film. Not only the growth of 3D Bragg peaks but also shifts of the peak positions, induced by the increase of the intra- and intergrain roughness, are simulated. The calculated spectra are compared with the experimental intensity profiles along the streaks measured together with a RHEED intensity oscillation during a homoepitaxy of Ag films.