The effect of ultrasonic irradiation on nucleation phenomena in the heat storage material CH3COONa center dot 3H2O was investigated by measuring the nucleation probability and induction time. The experimental results show that the concentration of CH3COONa should be operated in the optimum range for CH3COONa center dot 3H2O-selective crystallization. Furthermore, the nucleation probability increased and the induction time decreased with increasing energy input. Consequently, the optimum conditions of ultrasonic irradiation of CH3COONa center dot 3H2O are 53?wt-% CH3COONa, 3.0?W of energy input, and ?T = 1068?K.