We have observed x-ray intensity oscillation fringes around low index Bragg reflections in aYBa2Cu3Ox(YBCO) thin film. By employing a combination of crystal truncation rod interference fringe measurements and x-ray reflectivity using synchrotron radiation, we have been able to probe the structures of highly oriented [001] YBCO grown on [001]SrTiO3substrates. The results demonstrate the presence of a thin disordered surface layer, the excellent coherence between the YBCO surface and the film-substrate interface, the presence of a small(3.2×10−4)interfacial strain existing in the YBCO film. This strain exists close to the film-substrate interface and extends approximately 300 Å into the micron-thick film. Our results demonstrate that high quality detailed information can be obtained, nondestructively, from thin film superconductors of thicknesses typically used for microwave and other applications. ©1998 American Institute of Physics.