Abstract: Disclosed is a method of crystallizing .alpha.-L-aspartyl-L-phenylalanine methyl ester, which is a sweetening agent substituting sugar and commonly referred to as aspartame. An aspartame solution is sprayed and droplets of the solution fall downward. Cold airflow is blown to the falling droplets of the aspartame solution to cool them. The aspartame dissolved in the droplets crystallizes, and the slurry including the aspartame crystals and the remaining solution is collected for further processes. Also disclosed is a crystallizer for cooling crystallizing the aspartame solution with this method. The crystallizer has an atomizer for spraying the aspartame solution supplied thereto and an air inlet and an air outlet. The cold air is blown into the crystallizer through the air inlet, cools the droplets of the aspartame solution, and is discharged through air outlet.