Process for preparing single crystal

A process for preparing a single crystal comprising pulling up a single crystal by the Czochralski process and cooling it at a temperature not lower than 600.degree. C. at a reduced pressure or in vacuo wherein the cooling step of the pulled single crystal comprises lowering the temperature at a rate of up to 5.degree. C./min. to a temperature higher than 600.degree. C., keeping the same temperature for 1 to 2 hours, reducing the pressure at the same temperature at a rate of up to 60 Kg/cm.sup.2 /hr. to 1 to 3 kg/cm.sup.2, keeping the same pressure for 1 to 2 hours, lowering the temperature at the same pressure for 1 to 2 hours, lowering the temperature at the same pressure at a rate of up 5.degree. C./min. to a temperature not lower than 600.degree. C., reducing the pressure at the same temperature at a rate of up to 60 Kg/cm.sup.2 /hr. to atmospheric pressure, evacuating at a rate of up to 10 Torr/min. to a pressure of 10.sup.-2 Torr. or lower, and keeping the same vacuum for 1 to 2 hours.