Highly a-axis oriented epitaxial thin films of YBa2Cu3O7-delta a have been prepared by both a low temperature process and a Iso a self-tem plate technique. Films deposited at low temperature showed good crystallinity whereas films grown on a template exhibited a high transition temperature into the superconducting state. Detailed transmission electron microscopic investigations have been performed on these two kinds of a-axis oriented films. Significant differences have been found in the microstructures of these films. The dominant defects formed in these films are misoriented grains which mainly show a c-axis orientation. The origin of the nucleation of misoriented grains is attributed to surface defects of the substrate. No boundary between the template layer and upper layer could be distinguished for films made by the self-template process. A thin c-axis intermediate layer with a thickness of 2-5 unit cells was observed at the interface between the a-axis film and the SrTiO3 substrate for both kinds of films. The formation and influence of such an intermediate layer needs further study.