Ba0.6Sr0.4TiO3 thin films were epitaxially grown on (001) MgO substrates using pulsed laser ablation. Cross-sectional and plan-view transmission electron microscopy have been employed to study the microstructures and the interface behavior of the as-grown thin films. The 110-nm-thick Ba0.6Sr0.4TiO3 thin films have a flat surface and sharp interface. The entire thin film has a single-crystal cubic structure with an interface relationship of (001)Ba0.6Sr0.4TiO3//(001)MgO and <100>Ba0.6Sr0.4TiO3//<100>MgO with respect to the substrate. The 6.4% lattice mismatch between Ba0.6Sr0.4TiO3 and MgO was completely released at the interface by forming equally spaced misfit dislocations with a distance of ∼3.2 nm. High-resolution transmission electron microscopy investigation shows that the initial grown layer of the film is the TiO2 monolayer. The growth models of “substrate surface-terrace induced defects” for perovskite on a rock-salt system have been developed to understand the as-grown features where the conservative and nonconservative antiphase boundaries can be formed. © 2002 American Institute of Physics.