It was found from plan-view transmission electron microscopy (TEM) observations that c-oriented YBa2Cu3O7-delta (YBCO) films on LaAlO3, prepared by the magnetron sputtering technique had a single crystalline-like YBCO matrix, but always contained some precipitates of impurity phases. A large number of highly defective twin boundaries were also observed in all of these epitaxial films. Such highly defective twin boundaries are thought to be flux pinning centres and also paths for the diffusion of water into YBCO films. Therefore the films need to be passivated against attack from humid air.