The effects of the crystallinity and surface roughness of CeO2 buffer on R-plane Al2O3 substrate on the microwave surface resistance (R-s) of YBa2Cu3Oy (YBCO) thin alms are discussed. We estimated R-s from the transmission characteristics of the microstrip line resonator at 25 K and 6.7 GHz. X-ray diffraction (XRD) of theta-2 theta and phi-scan showed that CeO2 was completely (001)-oriented and in-plane aligned crystal. Fear CeO2 samples with different thicknesses were prepared using identical conditions except for the deposition time, The dependence of RI on CeO2 thickness was measured in the range from 10 mn to 200 nm. The value of R-s was minimum at CeO2 thickness of 100 ma The dependence of R-s vs CeOs thickness was similar to that of the amount of a-axis domains against the thickness The crystallinity of thin CeO2 was poor because the lattice was strongly strained by Al2O3. This affected the quality of the upper YBCO layer. In contrast, thick CeO2 had excellent crystallinity. However, for the thickness of more than 100 nm a drastic change in surface morphology was observed bg atomic force microscopy (AFM), a number of projections appeared on the CeO2 suface. These projections act as nucleation centers for the a-axis domains.