We present the results from the photoreflectance (PR) analysis of a set of molecular‐beam epitaxially grown silicon on sapphire films whose thicknesses ranged from 6 to 4000 nm. The strain undergone by the silicon films due to lattice mismatch at the silicon sapphire interface and to the difference in the thermal expansion coefficient of the two materials was investigated. To this purpose the position of the 3.4‐eV PR structure was monitored with bulk silicon as a standard. A shift of this structure toward lower energies was observed in the thinnest films. A drastic increase in elasticity is observed as the film thickness varies from 550 to 700 nm. This fact can be correlated with transmission electron microscope observations of the predominant defects in the films. Special attention was also devoted to the effects of native oxide on the films, that appeared to cause a shift of the 3.4 eV structure toward higher energies. This shift suggests that the native oxide layer is exerting a compressive strain on the silicon films.