Transmission electron microscopy has been used to investigate the microstructure of YBa2Cu3O7−x thin films deposited by Plasma-Enhanced Metalorganic Chemical Vapor Deposition. The films with the best superconducting properties of Tc ∼90K and Jc>106A/cm2 at 77K and OT were either Y-rich or both Y- and Cu-rich relative to the ideal 1:2:3 composition. In the Y-rich thin films, nanoscale yttria(Y2O3) precipitates have been identified. The precipitates with an average size of about 50Å and densities as high as 1024/m3 are uniformly distributed and highly oriented with respect to the matrix. Excess Cu in the Cu- and Y-rich films leads to formation of large CuO particles on the film surface but does not have any effect on formation of nanoscale Y2O3 phase within the films. The Y2O3 precipitates may act as flux pinning sites for high critical current densities.