High quality epitaxial YBa2Cu3O7−x(YBCO) superconducting thin films (0.3 &mgr;m thick) were grown on the closely lattice and thermal expansion matched substrate, LaAlO3, which has low dielectric loss. The YBCO layers were prepared,insitu, by a microwave plasma‐enhanced metalorganic chemical vapor deposition process. The films, which had mirror‐like smooth surfaces, were deposited at a substrate temperature of 730 °C with a partial pressure of 2 Torr of N2O. The electrical resistance and magnetic susceptibility versus temperature of the as‐deposited films show metallic behavior in the normal state and sharp superconducting transitions withTc(R=0) of 88 K. Critical current densities measured on patterned bridges were 5×105A/cm2at 78 K for the films deposited on LaAlO3. X‐ray diffraction measurements indicate that films grow epitaxially in the plane of the substrate with axis perpendicular to the substrate surface.