We consider hemoglobin (Hb) components for possible use as cathode electrode catalyst in polymer electrolyte fuel cells (PEFCs). We choose iron-porphyrin (FeP) and (imidazole)FeP [(Im)FeP] as a representative heme complex of the Hb active site, and investigate their catalytic activities for O₂ dissociation using it ab initio calculations based on density functional theory (DFT). The imidazole ligand suppresses the donation/back-donation of electrons between Fe and O₂, resulting in the weakening of the Fe-O₂ bond and the strengthening of the O-O bond. Therefore, the catalytic activity of FeP is higher than that of (Im)FeP. [DOI: 10.1380/ejssnt.2004.226]