Disclosed herein is a process for producing a (2S,5S)-2,5-diamino-1,6-diphenyl-3-hexanone derivative represented by the formula (1): ##STR1## wherein R.sup.1 means a lower alkylcarbonyl group, a halogen-substituted lower alkylcarbonyl group, an arylcarbonyl group or a lower alkoxycarbonyl group, and R.sup.2 and R.sup.3 are identical with or different from each other and denote individually an aryl group or an alkoxy-substituted aryl group, which comprises subjecting a (2S)-2,5-diamino-1,6-diphenyl-4-hexen-3-one derivative represented by the formula (2): ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 have the same meaning as defined above, to a hydrogenation reaction in the presence of a transition metal-containing catalyst.