To obtain optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (1) via optical resolutions by replacing and preferential crystallization, the racemic structure of (2RS,3SR)-1 hydrochloride [(2RS,3SR)-1·HCl] was examined based on the melting point, solubility, and infrared spectrum. (2RS,3SR)-1·HCl was indicated to exist as a conglomerate at room temperature, although it forms a racemic compound at the melting point. When, in optical resolution by replacing crystallization, L-phenylalanine methyl ester hydrochloride (L-2) was used as the optically active co-solute, (2R,3S)-1·HCl was preferentially crystallized from the supersaturated racemic solution; the use of D-2 as the co-solute afforded (2S,3R)-1·HCl with an optical purity of 95%. In addition, optical resolution by preferential crystallization was successfully achieved to give successively (2R,3S)- and (2S,3R)-1·HCl with optical purities of 90—92%. The (2R,3S)- and (2S,3R)-1·HCl purified by recrystallization from 1-propanol were treated with triethylamine in methanol to give optically pure (2R,3S)- and (2S,3R)-1.