PURPOSE: To obtain one isomer of phenylpropionic acid useful as a drug economically, efficiently and in high purity, by dissolving an isomer mixture of a phenylpropionic acid in a solvent and carrying out crystallization.

CONSTITUTION: An isomer mixture of a phenylpropionic acid shown by the formula (R is H, alkyl, alkenyl or aryl) is dissolved in a solvent (e.g. hexane, heptane, methanol, acetone, ethyl acetate, etc.). The concentration of the solution is ordinarily up to about 15wt%. After the dissolution, crystal is precipitated by methods of (i) changing temperature of the solution, (ii) adding a solvent such as water, etc., (iii) distilling away part of the solvent, etc., to give the aimed compound. When purity of the crystal is insufficient, the crystal is redissolved in the solvent and crystallization is repeated to improve purity.