The production of isophthalic acid (IPA) from the oxidation of m-xylene (MX) with air can be sped up by Co(II) and phosphotungstic acid (HPW) loaded on activated carbon. In this paper, activated carbon is modified with a ZnCl2 solution to improve the catalytic performance of the HPW@C catalyst. The experiments indicate that the optimum modification condition is the carbon immersed in a 0.10 mol L-1 ZnCl2 solution for 6 h at the temperature of 40 degrees C followed by being carbonized at 600 degrees C for 4 h. The characterization proves that the ZnCl2 modification leads to the increase of surface area and acidic groups on the activated carbon. The MX conversion catalyzed by the HPW@C prepared from the carbon modified is 7.0 % over that by the HPW@C from the original carbon and the IPA produced by the modified sample is 78.9 % over that produced by the original one.