In situ two-step hot filament chemical vapor deposition (HFCVD) was developed to synthesize diamond films on aluminum oxide (Al 2 O 3 ) substrates. The first step at higher temperature leads to precipitation of diamond-like carbon (DLC) and aluminum carbide (Al 4 C 3 ) phases on the substrate surface, which promote enhanced nucleation of diamond growth during the subsequent second step of deposition. The presence of carbide phase in the interface region is found to lead to 100 oriented growth of diamond as well as enhanced adhesion properties.