The structure of pentacene thin films grown by supersonic molecular beam deposition on two Ag(111) single-crystal surfaces with different average step distances ( similar to 400 angstrom and > 2000 angstrom) have been studied by low-energy atom diffraction. While the initial stage of the growth is similar on the two different surfaces, regardless of the pentacene kinetic energy and substrate temperature, thicker films show different structural and thermal properties. The ultrathin-film phase has the same structure on both surfaces, showing, however, much larger domain sizes on the substrate with larger average terrace width. In spite of the inferior quality of the ultrathin layer grown on the narrower terraces, upon continuing the growth, a well-ordered multilayer structure is obtained. This, however, occurs in a narrow range of deposition conditions, i.e., at a relatively high kinetic energy of the molecules (similar to 5 eV) and a low substrate temperature (200 K). On the other, almost ideally flat surface, the order of the pentacene multilayers is, surprisingly, much poorer in spite of the facts that for this surface we have measured an unprecedented 90% of He atom reflectivity and that the measured width of the diffraction peaks of the first monolayer was the narrowest we have ever measured for an organic thin film.