Disclosed herein is a process for producing highly pure terephthalic acid which forms polyesters of a particularly favorable color tone, the process comprising the steps ofoxidizing p-xylene, thereby obtaining a mixture containing terephthalic acid,subsequently further oxidizing the thus obtained mixture at not higher temperature than that of the first oxidation step,still further oxidizing the reaction mixture of the second oxidation step at a higher temperature than that of the first oxidation step andtreating the reaction mixture obtained by the third step of oxidation by bringing thereof into contact with molecular oxygen.