The diamond film deposited on the cobalt cemented tungsten carbide substrate by the hot filament assisted chemical vapor deposition (CVD) method was studied by scanning electron microscopy, Auger electron spectroscopy, and transmission electron microscopy Surface pretreatment for the substrate is essential to the film quality and adhesion strength of the CVD grown diamond film. The presence of cobalt on the substrate surface is detrimental to the film growth. According to Auger studies, a carbon layer, possibly amorphous carbon, with a thickness less than 120 angstrom was present at the interface between the diamond film and the cobalt cemented tungsten carbide substrate. Cross-sectional transmission electron microscopy revealed that the diamond crystals can be grown directly from a polycrystalline tungsten carbide substrate without forming the interfacial carbon layer. The cobalt in the cemented tungsten carbide may be ascribed to the formation of the interfacial amorphous carbon layer for the hot filament assisted CVD grown diamond film.