A process for preparing optically active 4-hydroxy-2-pyrrolidone comprising asymmetrically hydrogenating an N-substituted-4-amino-3-oxobutanoic ester represented by formula (I): ##STR1## wherein R.sup.1 represents a benzyloxycarbonyl group, the benzene ring of which may be substituted; and R.sup.2 represents a lower alkyl group having 1 to 4 carbon atoms, in the presence of a ruthenium-optically active phosphine complex as a catalyst to obtain an optically active N-substituted-4-amino-3-hydroxybutanoic ester, deblocking, and cyclizing the ester. A series of the reactions can be carried out in one pot. Optically active 4-hydroxy-2-pyrrolidone is obtained in high yield with high optical purity, and is useful in the synthesis of carbapenem antibiotics