With microwave irradiation, phthalic anhydride was obtained with a high yield of 67 mol% at ca. 563 K by the oxidation of o-xylene with air over V2O5/SiO2 system. It is found that dispersion of V2O5 on SiO2 is more homogeneous when the catalyst is prepared by microwave irradiating method. In the microwave catalytic process the optimum reactor bed temperature of the title reaction decreases to 563 K (653 K in the conventional process). The effects of the microwave electromagnetic field on the catalysts are discussed.