Electrical resistance measurement of optimal doped YBCO under pressure

High pressure effect on nearly optimal doped high $T_{C}$ cuprate superconductor YBa$_{2}$Cu$_{3}$O$_{7-x}$ was studied by the electrical resistance measurements up to about 30 GPa. Superconducting phase of YBa$_{2}$Cu$_{3}$O$_{7-x}$ in pressure-temperature phase diagram was confirmed. $T_{C}$ has the broad maximum at about 8 GPa and then decreases with pressure and disappears at the pressure between 23 GPa and 25 GPa. In higher pressure region, the resistance shows upturn below about 60 K, indicating the possibility of crossover on YBa$_{2}$Cu$_{3}$O$_{7-x}$ from superconductor to semiconductor at about 24 GPa