PURPOSE: To obtain a high purity aromatic dicarboxylic acid for the monomer of a thermotropic liquid crystal polymer, etc., in a high yield by using 4,4'-diacyl derivative which is easily available as a raw material and oxidizing it with oxygen at a specified temperature in the presence of a catalyst and a solvent.

CONSTITUTION: A diacyl compound given by formula I (R1, R2 are each 1-4 alkyl allowed to be substituted; X is a chemical bond, O, S) (example: 4,4'- diacetyldiphenyl, etc.) is oxidized with oxygen at a temp. of 20-250°C, especially 90°-boiling point of reaction mixture, in the presence of a catalyst (preferably a salt of vanadium, manganese, iron, or cobalt) and a solvent (preferably a 1-5C carboxylic acid) to obtain aromatic dicarboxylic acid given by formula II (example: diphenyl-4,4'-dicarboxylic acid, etc.). A compound of formula I is easily available by Friedel-Crafts acylation.