In general, it is said that a content in a reaction vessel for the synthesis of urea from ammonia and carbon dioxide under high temperature and pressure is separated into two phases of melted solution and vapor.(1) But no one, so far as I know, has succeeded in analyzing plainly the equilibrium between the two phases in spite of the importance of industrial production of urea. The writer has studied the equilibrium between the two phases of the CO2-NH3-Urea- H2O system under high temperature and pressure.