The deformation of epitaxial silicon films grown on single‐crystal sapphire has been measured. The range of silicon thicknesses was between 1 and 46 &mgr; and the range of sapphire thicknesses was between 127 &mgr; (0.005 in.) and 508 &mgr; (0.020 in.). The deformation of the films was found to be proportional to silicon film thickness and decreased with increasing sapphire thickness. For sapphire disks of 0.95 cm (⅜ in.) diameter and 250 &mgr; (0.010 in.) thickness the proportionality constant was unity. The stresses in the deformed silicon films have been estimated on the basis of linear beam theory and were of the order of 109to 1010dyn/cm2. The silicon films were under compression and the sapphire was under tension. No physical damage to the films was observed under these stresses, but these stresses should be sufficient to affect the electrical properties of the films.