Optically active diphenylalanine compounds may be conveniently prepared in a good yield by reacting a diphenylalanine compound represented by formula (1) with an optically active amine compound represented by formula (2) in the presence of an organic solvent to give a diastereomeric salt represented by formula (5) and then treating the diastereomeric salt under acidic conditions to give an optically active diphenylalanine compound represented by formula (3): ##STR00001## wherein each symbol is as defined in the specification.