Abstract: The solubilities of the ternary system, Y(NO₃)₃ ·3H₂O-18C6-CH₃COCH₃, at 18℃ have been determined by the semimicro method to study of phase equilibrium. The results indicated that there is only one solubility curve for this ternary system, it correspounds with the solid phase of the complex CY(NO₃)₃ ·18C6·3 H₂O]. We have not found any other complex species having different ratios, Y(Ⅲ) crown.The behavior of water for this system in equilibrium has been examined. The results are. the mole ratio [H₃O/Y(NO₃)₃], no matter whether it is in liquid phase or solid phase, is always 3:1.The relative stable complex, Y (NO₃)₃ ·18C6·3H₂O, at first has been obtained by us. The composition of this complex has been examined by chemical analysis and infrared spectra.