The c-axis oriented 1-UC YBa2Cu3O7-x (YBCO) layer was deposited on a PrBa2Cu3O7-x (PrBCO) buffer layer on SrTiO3 by reactive coevaporation. Deposition of the PrBCO buffer layer of several unit cells followed by a 1-UC YBCO layer was interrupted for 20 s at every 1-UC layer by monitoring RHEED oscillations in order to improve the surface flatness. Atomic force microscopy (AFM) and cross-sectional TEM images of ultrathin films indicate that the conduction path of the 1-UC YBCO layer parallel to the a - b plane will be kept over the sample, suggesting that its transport properties reflect the intrinsic nature. The 1-UC YBCO layer which has CuOx at the terminating layer is nonsuperconductive till BaO layer is deposited on it. This means that the completion of the charge reservoir block BaO-CuOx-BaO is necessary for carrier-doping in CuO2 conduction plane. The carrier concentration enough to give rise to superconductivity should be supplied from respective charge reservoir blocks neighboring to the CuO2 bilayer, one of which is shared with PrBCO of the buffer layer. In the resistive transition of the 1-UC YBCO layer, the mean field transition temperature T(mf) remains still high as 82 K but the zero resistive transition temperature is about 35 K. We present transport properties which prove Kosterlitz-Thouless transition in the 1-UC YBCO layer.