The influence of compressive strain on the linear electro-optic (EO) effect in c-domain (BaxSr1-x)TiO3 (BST) thin films was theoretically investigated using a phenomenological thermodynamic model. We considered that the c-domain BST thin films were grown on cubic substrates and an electric field was applied parallel to the direction of spontaneous polarization. To estimate the EO coefficients, we considered not only the intrinsic EO effect but also the elasto-optic effect by converse piezoelectric response. We found that the EO response is mostly determined by the difference in temperature from the ferroelectric-to-paraelectric phase transition boundary modified by the strain, and the elasto-optic effect appreciably affects the overall EO coefficient when the intrinsic EO effect is weak. (C)2019 The Ceramic Society of Japan. All rights reserved.