A process for preparing an N-acyltetrahydroisoquinoline represented by formula (II) ##STR1## wherein A represents a phenylene ring substituted with a hydroxyl group, a lower alkoxy group, an acetoxy group, or a benzyloxy group; R represents a hydrogen atom, a lower alkyl group, or a phenyl group; and X represents a hydrogen atoms, a phenyl group, or a phenyl group substituted with a hydroxyl group, a lower alkoxy group, or an acetoxy group, which comprises asymmetrically hydrogenating an N-acyl-1-methylenetetrahydroisoquinoline or N-acyl-1-benzylidenetetrahydroisoquinoline represented by formula (I) ##STR2## wherein A, R, and X are as defined above, in the presence of an optically active ruthenium-phosphine complex as a catalyst. The process exclusively and efficiently provides a useful isomer of the N-acyltetrahydroisoquinoline of high purity which is useful as an intermediate for synthesizing isoquinoline type alkaloids as pharmaceuticals without involving optical resolution of a racemate.