The iron porphycene with two propionates at the peripheral positions of the framework was incorporated into the heme pocket of horseradish peroxidase. In the presence of hydrogen peroxide, the ferric iron porphycene was smoothly converted into the iron(IV)-oxo porphycene pi-cation radical species, which was confirmed by the appearance of a band around 800 nm in the UV-vis spectrum. The protein with the iron porphycene showed a 10-fold higher reactivity for the thioanisole oxidation when compared to the native protein. In contrast, the guaiacol oxidation proceeded with similar reaction rates in both proteins. The kinetic analyses indicated that the ferric porphycene in the protein more slowly reacts with hydrogen peroxide than the native heme, whereas the high oxidation states show higher reactivities during oxidations of an organic substrate. The formation of the iron(IV)-oxo species of porphycene and its reactivities in the hemoprotein matrix are demonstrated.