The present invention discloses a process which comprises selectively hydrolyzing one enantiomer of racemic mixtures of an N-substituted .beta.-amino acid alkyl ester or N-substituted 2-homopipecolic acid ester represented by the formula (I): ##STR00001## wherein Ar, R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are the same as defined in the specification, in the presence of a hydrolase to form an optically active ((R) or (S))--N-substituted .beta.-amino acid or optically active ((R) or (S))--N-substituted 2-homopipecolic acid represented by the formula (II): ##STR00002## and simultaneously to obtain an unreacted optically active ((S) or (R))--N-substituted .beta.-amino acid alkyl ester or unreacted optically active ((S) or (R))--N-substituted 2-homopipecolic acid ester represented by the formula (III): ##STR00003## which has a reverse steric absolute configuration to that of the compound represented by the formula (II).