Vapor‐deposited films prepared under low adatom mobility conditions commonly exhibit columnar structures. We present the scanning electron microscopy study of Cufilms with a thickness around 3.7 μm deposited on rough Al2O3ceramic substrates and smooth SiO2 substrates. The Cufilms were deposited using the partially ionized beam deposition technique under the same deposition conditions. The filmsdeposited on the rough substrates have a typical columnar structure while those deposited on the smooth substrates are noncolumnar. The complete difference in physical structures is due to the difference in the substrate surface roughness. The results indicate that in any structure zone model for the physical structure of the film the substrate surface roughness is an important parameter and cannot be neglected. It is reasoned that the adatom surface diffusion length, which is determined by the deposition conditions, serves as a critical measure for the length scale of surface roughness. At the initial stage of the film growth, the columnar structure develops only for the roughness with a length scale longer than adatom surface diffusion length.