A process for preparing carnitine which comprises asymmetrically hydrogenating a .gamma.-halogeno-.beta.-keto ester represented by formula (I): ##STR1## wherein X represents a chlorine atom or a bromine atom; and R represents a lower alkyl group, in the presence of a ruthenium-optically active phosphine complex represented by formula (II), (III) or (IV):Ru.sub.2 Cl.sub.4 (L).sub.2 (C.sub.2 H.sub.5).sub.3 N (II)Ru(OCOR.sup.2).sub.2 (L) (IV)RuX.sub.2 (L) (V)wherein L represents 2,2'-bis(di-p-R.sup.1 -phenylphosphino)- 1,1'-binaphthyl of formula (III): ##STR2## wherein R.sup.1 represents a hydrogen atom, a methyl group, or a t-butyl group;R.sup.2 represents a lower alkyl group or a trifluoromethyl group; and X is as defined above,as a catalyst at a temperature of from 70.degree. to 150.degree. C. to obtain an optically active alcohol represented by formula (VI): ##STR3## wherein X and R are as defined above, and then reacting the optically active alcohol as obtained with trimethylamine without isolation, is disclosed.