The monooxygenese activity of cytochrome P450 is successfully introduced into myoglobin by rational design of its active site. Introduction of an aromatic ring, tryptophan, near the heme by site-directed mutagenesis resulted in the hydroxylation of tryptophan at the C6 position by using an almost stoichiometric amount of H₂O₂. We also altered the substrate specificity of H₂O₂-dependent P450 by employing a simple substrate trick. Although P450_BSβ exclusively catalyzes peroxygenation of long-alkyl-chain fatty acids, oxidation of non-natural substrates such as styrene, ethylbenzene, and 1-methoxynaphthalen are catalyzed by P450_BSβ in the presence of decoy molecules having a carboxyl group. Advantageously, the substrate specificity of P450_BSβ can be altered by simply adding the decoy molecule without replacing any amino acid residues. Moreover, the stereoselectivity can be controlled by changing the structure of the decoy molecule. The crystal structure analysis of the decoy molecule bound-form of P450_BSβ shows that P450_BSβ accepts the decoy molecule, whose carboxylate is located at the same position to that of long-alkyl-chain fatty acid.