Résumé : C-axis oriented epitaxial YBa2Cu3O7-δ thin films ( Tc of 88 K and Jc of 107A/cm2 at 77K) were deposited on (100) MgO and SrTiO3 substrates using magnetron sputtering. Their structural properties, studied by x-ray diffraction, scanning electron microscopy, atomic force microscopy, Rutherford backscattering, and transmission electron microscopy, will be reported. The TEM study indicates a high density of nanometer-sized Y2O3 precipitates in these films, which can act as effective pinning centers and lead to the very large critical current densities observed in these films.