In recent years, synthesizing diamond film from gas phase is expected to be a new method of surface hardness improvement. Since the diamond film has a rough surface, it may be necessary to be polished. But the adhensive strength between the film and the substrate is too weak to endure the force produced by the conventional polishing method using diamond powders, so some new polishing method is needed. In this study, using the thermo-chemical reaction between diamond and iron, a polishing apparatus is manufactured for trial. A diamond film deposited on a 7 × 7× 4 mm silicon substrate by microwave plasma CVD is used as a workpiece, an iron plate heated to 810°C is used as a polishing plate, the polishing pressure of 60 kPa is much smaller than that of the conventional method, and the polishing experiment is performed in a hydrogen gas atmosphere. After 2 hours' polishing, the diamond surface becomes smooth without any exfoliation.