Superconducting thin films of YBa2Cu3Ox (YBaCuO) and NdBa2Cu3Ox (NdBaCuO) were grown by mist microwave-plasma chemical vapor deposition (MPCVD) using a CeO2 buffer layer on a MgO (001) substrate. In this method, the CeO2 buffer layer was deposited on the MgO (001) substrate at 1173 K by MPCVD. YBaCuO and NdBaCuO films were then grown at 1073 K and 1223 K, respectively. The T-c (zero resistance) values of the YBaCuO and NdBaCuO films obtained with a CeO2 buffer layer were 90.1 K and 94.1 K, respectively, about 10 K higher than those without a CeO2 buffer layer. The surface roughness of the films was less than 5 nm in each case. The interface between the substrate and the grown layer was confirmed to be extremely sharp by Auger profile analysis.