Perovskite-based BaNb0.2Ti0.8O3 (BNTO) thin film, grown by computer-controlled laser molecular beam epitaxy on SrTiO3 (001) substrate, was investigated by means of high-resolution electron microscopic (HREM) imaging, high-angle annular dark-field (HAADF) imaging, and X-ray energy dispersive spectrometric (XEDS) line-scanning in a transmission electron microscope. The microstructure of the BaNb0.2Ti0.8O3 film is clarified in terms of various domains due to tetragonal characteristics. Compositional fluctuation was observed in the various domains and domain boundaries. The misfit strain and element diffusivity during the film growth together with inherent tetragonal characteristics are discussed as a possible mechanism for the formation and distribution of various domains. (C) 2008 Elsevier B.V. All rights reserved.