Ultrathin films of YBa₂Cu₃O₇(YBCO) were grown on nonsuperconducting PrBa₂Cu₃O₇(PrBCO) buffer layers by reactive evaporation monitoring with RHEED oscillations. The one-unit-cell-thick layer of YBCO covered by a PrBCO layer exhibits a superconducting transition with the onset temperature at 70K. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers. The covering PrBCO layer has been found to provide the underlying YBCO layer with the carriers.The resistive transition for one-unit-cell-thick YBCO layer was independent of the magnetic fields parallel to a-b plane.