The oxidation process of an HfC(100) surface has been investigated by photoemission spectroscopy utilizing synchrotron radiation. When the HfC(100) surface is exposed to O₂ at room temperature, the C atoms in the surface region are depleted, and the substrate's Hf atoms are oxidized by stepwise reactions and are finally oxidized to form an HfO₂-like layer at high coverages (> 10 L). When the surface exposed to 10 - 50 L of O₂ is heated, the work function decreases with increasing heating temperature. The work function reaches its minimum value, which is lower than that of the clean surface by 1.2 eV, by heating the surface exposed to 10 L (50 L) of O₂ at 1000 - 1100°C (1100- 1200°C). The oxidized layer which gives the minimum work function is proposed to be mostly composed of oxycarbides, and a part of the surface is proposed to be covered by an HfO-like layer. [DOI: 10.1380/ejssnt.2006.219]