To investigate the influence of CeO2 buffer layer on the subsequent YBa2Cu3Ox (YBCO) growth superconducting YBCO films were deposited on two types of substrates: as-grown or annealed CeO2 buffered sapphire. The 0 0 2 CeO2 rocking curves recorded from the as-grown CeO2 layer have a two-component shape. Annealing of the CeO2 layers improves the crystalline quality of the layers and it modifies the two-component shape of the rocking curves. The crystallinity of the YBCO films was characterized by X-ray diffraction and transmission electron microscopy analyses. Microwave investigation of both types of films showed significant difference in temperature dependence of microwave losses between the YBCO films grown on the as-deposited CeO2 (gradual transition to the superconducting state) and on the annealed CeO, layers (simple and sharp transition to the superconducting state). Our results show that used annealing procedure improves the properties of the YBCO films, reduces the presence of high angle grain boundaries and diminishes the dispersion of their critical temperature.