We investigated change of crystal orientation of YBa2Cu3O7 thin films on CeO2/Al2O3 with respect to thickness of YBa2Cu3O7 films using X-ray reflectivity and diffraction. We observe that transition from c-axis orientation to a-axis orientation normal to surface occurs abruptly and the thickness at the transition is similar to900 Angstrom. The density of the a-axis oriented layer obtained from the fitting of X-ray reflectivity is slightly smaller than that of bulk value though the density of c-axis oriented layer is nearly identical with bulk value. The data also show that the a-axis oriented layer shows in-plane 90degrees b-c twinning. (C) 2002 Elsevier Science B.V. All rights reserved.