The present invention provides a method of producing easily and in a high yield a threo-2-hydroxy-3-nitrobutanoic acid compound or a mixture of a threo- and an erythro-2-hydroxy-3-nitrobutanoic acid compounds, this mixture having an erythro compound:threo compound ratio of about 1:1, which method includes reacting, in a solvent, a 2-hydroxy-3-nitrobutanoic acid compound having the formula (I) 1 wherein R is optionally substituted alkyl or optionally substituted aryl, with not less than 1 equivalent of a base to isomerize (i) the erythro compound into the threo compound, or (ii) the threo compound into the erythro compound, and a method for separating the erythro and the threo compounds easily and in a high yield. The separation method includes reacting the mixture of threo- and erythro-compounds with not more than 1 equivalent of potassium carbonate per threo compound in a solvent. According to the method of the present invention, useful pharmaceuticals can be produced easily in a high yield.