A process for preparing N-substituted amino acid esters having the formula (I): ##STR1## wherein R.sub.1 and R.sub.4, the same or different, are an alkyl, aralkyl, cycloalkyl or aryl group; and R.sub.2 and R.sub.3, the same or different,are an alkyl, aralkyl, aryl, heterocycle-alkyl, aminoalkyl or guanidylalkyl, by the reaction of .alpha.-amino acid esters with .alpha.-substituted carboxylic acid esters, under the condition of substantially free from solvent, which can afford their optical isomers in a good yield.