High quality LiNbO 3 (LN) epitaxial films have been grown onto (001)Al 2 O 3 substrates by pulsed laser deposition. The deposition conditions have been optimized using experimental designs to promote the growth of highly (001)-oriented, phase pure and light-guiding LN films. The structural and nanostructural properties have been investigated by X-ray diffraction reciprocal space mapping and the guiding properties have been investigated by m-line spectroscopy and measurements of the light propagation losses. In particular it is shown that the film composition, the state of strain, the film thickness, the film roughness, the lateral extension of the crystallites building up the film as well as the mosaicity can be determined by a careful examination of the X-ray reciprocal space maps associated with simulation of the diffracted intensity distribution in reciprocal space. The guiding properties have been correlated with the nanostructural properties of the films: whereas light guiding has been clearly observed in single crystal-like films, the existence of a mosaic structure, made of nanocrystalline domains, is shown to be detrimental to the guiding properties.