A process for preparing an optically active 4-mercapto-2-pyrrolidone derivative of the formula: ##STR1## wherein a group of the formula: --SR.sup.1 is a protected or unprotected mercapto group, and the group of the formula: .dbd.NR.sup.2 is a protected or unprotected imino group, which comprises subjecting racemic 4-amino-3-mercaptobutyric acid or a salt thereof to optical resolution by using 1-(2,3,4-trichlorophenyl)ethanesulfonic acid, followed by subjecting the product to cyclization reaction after protecting the functional groups thereof, if necessary, and further optionally removing the protecting groups therefrom. The present process is industrially advantageous than conventional processes for preparing optically active 4-mercapto-2-pyrrolidone derivatives which are useful as an intermediate for various medicines such as carbapenem antibacterial agents.