In this study, the catalytic oxidation of o-xylene was used to produce phthalic anhydride in the presence of molecular oxygen in a fixed bed reactor. In this study, the V 2 O 5 /ZnO/ ZrP catalyst was synthesized and identified by techniques such as TEM, FE-SEM, TGA, XRD, ICP, FT-IR, XPS, BET. Then factors such as temperature, space velocity (WHSV), flow rate of oxygen, percent of promoter and vanadium metal were optimized for this reaction in order to achieve the highest conversion rate and selectivity. Finally, 84% selectivity to phthalic anhydride and 100% conversion of o-xylene in the presence of 15wt% of vanadium and 1wt% of zinc metal at 410 0 C and a flow rate of 9.5 ml/min of oxygen gas and a WHSV of 0.73 h -1 was obtained.