Optically active .beta.-phenylalanine compound may be prepared in an industrially advantageous manner by reacting an N-acyl-.beta.-phenylalanine compound with a specific optically resolving agent to effect an optical resolution by formation of diastereomer salts, and removing the optically resolving agent from each diastereomer, to give an optically N-acyl-.beta.-phenylalanine compound. Deacylation may be further carried out to obtain an optically active .beta.-phenylalanine compound.