Thin films used for optical components require improvements in their mechanical properties and long-term stability. We examine the influence of substrate weathering on optical thin films. The light scattering of TiO2 thin films increased as the substrate is weathered. The light scattering of the TiO2 thin film formed by ion-assisted deposition (IAD) is smaller than that of the TiO2 thin film formed by electron beam deposition (EBD).