The microstructures of (A) near stoichiometric, (B) Y-rich, and (C) Y- and Cu-rich YBa2Cu3O7−x thin films have been studied by high-resolution transmission electron microscopy. The films were deposited on (100) LaAlO3 by plasma-enhanced metalorganic chemical vapor deposition. In near stoichiometric films, microstructural features similar to those of thin films deposited by other techniques have been observed. These features which include epitaxial growth with the c-axis perpendicular to the substrate, twin boundaries on (110) planes, and stacking faults on (100) and (001) planes were also present in the off-stoichiometric materials. In Y-rich thin films, yttria (Y2O3) precipitates with an average size of about 5 nm have been identified in the matrix. The precipitates are uniformly distributed, have a high density as large as 1024/m3, and are highly oriented with respect to the matrix. In Y- and Cu-rich thin films, CuO particles up to 1 μm in size were observed on the surfaces of the films. The observed microstructural features were similar to those of the Y-rich materials.