Abstract: Provided is a process for separating (S)- or (R)-2-(3-benzoylphenyl)propionic acid from a mixture of (S)- and (R)-2-(3-benzoylphenyl)propionic acid, which comprises (1) reacting the mixture of (S)- and (R)-2-(3-benzoylphenyl)propionic acid with (S)- or (R)-3-methyl-2-phenylbutylamine in a suitable solvent to form a diastereomer salt of (S)-2-(3-benzoylphenyl)propionic acid with (S)-3-methyl-2-phenylbutylamine or a diastereomer salt of (R)-2-(3-benzoylphenyl)propionic acid with (R)-3-methyl-2-phenylbutylamine; (2) separating the diastereomer salt from the reaction mixture; and (3) liberating the separated diastereomer salt to give (S)- or (R)-2-(3-benzoylphenyl)propionic acid. According to the present process, it is possible to obtain optically active (S)- or (R)-2-(3-benzoylphenyl)propionic acid having 99% or more of high optical purity in a high yield. Thus, the present process is a practical and efficient process which can simplify purification steps and can be applied for an industrial production process.