PURPOSE: A naphthalene oxidation catalyst is prepared by adding potassium sulfate to a titanium dioxide-vanadium pentoxide and supporting them on a nonporous inert carrier, thus increasing its selectivity, resulting in increase in the total yield.

CONSTITUTION: In the production of phthalic anhydride or a mixture therof with naphthoquinone by the gas-phase oxidation of naphthalene with a molecular oxygen-containing gas, as a catalyst is used the one prepared by supporting a mixture of active constituents consisting of 63W90wt%, preferably 65W80wt% of titanium dioxide, 5W30wt%, preferably 15W30wt% of vanadium pentoxide and 3.5W7wt%, preferably 4W7wt% of potassium sulfate on a nonporous and inert carrier, preferably a carrier with ring structure, especailly lessingite ring structure, because they develops noteworthy effect, when a large amount of potassium sulfate is added.