High quality, very flat Nd1.2Ba1.8Cu3Oy thin films have been grown by sputtering on TiO2, terminated SrTiO3 (100) single crystals and their surface structure have been determined by using Grazing Incidence X-ray Diffraction (GIXD) technique, employing synchrotron radiation. A 52 unit cells film presents a double terminated surface composed by a complete and ordered Cu(1)-O plane and a disordered Cu(l)-0 plane partially covered by a BaO layer, in full agreement with scanning tunneling microscopy data. A preliminary comparison between the structure of 8 and 52 u.c. NdBCO films will be presented. (c) 2005 Elsevier B.V. All rights reserved.