We investigated the ferroelectric characteristics of an epitaxial perovskite ZnSnO3 thin film on a (111) SrRuO3/(111) SrTiO3 substrate fabricated by pulsed laser deposition. We confirmed that the ZnSnO3 thin film was epitaxially grown on the substrate, forming large terraces on the surface of the ZnSnO3 thin film. The ZnSnO3 thin film exhibited a high ferroelectric polarization of similar to 47 mu C/cm(2), which was further supported by first-principles calculations.