Quantum voltages of an array of YBa 2 Cu 3 O 7 bicrystal junctions were calibrated against a programmable Josephson array voltage standard. We demonstrated that steps of the current-voltage characteristic of an array of bicrystal junctions at voltages of about 9 mV were flat over the current range of about 80 A to within six parts in 10 8 . The coincidence of quantum voltages on the array of high-temperature superconductor junctions at 64 K and the reference voltage on the array of niobium junctions at 4.2 K was measured with an uncertainty of two parts in 10 8 . With the same uncertainty, we revealed the coincidence of the Josephson constant K J ϵh/2e in YBa 2 Cu 3 O 7 and in metallic superconductors.