A simple and environmental-friendly procedure to synthesize 2-methyl terephthalic acid by selectively oxidizing 2, 4-dimethylbenzoic acid was reported. 2,4- Dimethylbenzoic acid reacts in alkalic solution with potassium permanganate (KMnO4) to afford 2-methyl terephthalic acid. The reaction is highly selective and takes only 5 hours. The chemical structure of the product was confirmed by FT-IR, 1H-NMR and 13C-NMR spectroscopy. The pathways of the procedure and the mechanism of the selective oxidation reactions were discussed.