Diamond films were grown heteroepitaxially on iridium (100) substrates by microwave plasma-assisted chemical vapor deposition (MPCVD) using methane gas as the carbon source. The iridium substrate, which was formed on a MgO (100) substrate by means of sputtering at 850 °C, was treated by imposing a negative bias between -150 and -200 V for 15 min. Methane concentration and substrate temperature were maintained at 3° and 650–740 °c, respectively. At a substrate temperature of 740 °C, diamond particles were formed at a population density of (0.15- 1.5)x108 cm-2, and most of them were oriented to MgO (100). After a further reaction for 1 h under conditions which were optimized for diamond growth, the oriented diamond particles were coalesced, and islands of (100) diamond were formed.