A very smooth surface film of c-axis oriented YBa2Cu3Ox (YBCO) with roughness of less than monomolecular layer over 10µmx10µm, free of precipitates, has benne obtained by atomic layer-by-layer metalorganic chemical vapor deposition (MOCVD) on a SrTiO3 substrate at 650°C. A very large terrace length of 0.3-0.5µm may be due to the enhanced migration of growing species on the surface. The result of an attempt to prepare YBCO films with a larger terrace width surface using NdGaO3 substrates is discussed. The correlation between boulder formation and dislocations in the substrate is clarified and methods for eliminating boulders are proposed. Very high superconductivity critical current densities of 3x107A/cm2 at 4.2K and 3x106A/cm2 at 77K have been obtained.