Studies of the resistivity and Hall effect inn‐type (221) Si/(112¯2) Al2O3films have shown that the electron mobility is a strong function of azimuthal direction in the plane of the film. Typical variations in mobility between high‐ and low‐mobility directions are approximately 40%. The anisotropy in mobility is explained on the basis of piezoresistance effects in Si, due to stresses induced by the differential thermal contraction between the Si and the Al2O3substrate.