During the synthesis of diamonds, there exists at high pressure and high temperature a thin metallic film covering the growing diamond. In this study, the relationship between the microstructures of the diamond/metallic film interface and the diamond growth mechanism is investigated using field emission scanning electron microscopy, energy dispersive spectrum, etc. The results show that the film covering diamond (100) facet consists of pyramid and tetragonal shaped nanocrystal particles, while serrate growth steps are presented on the surface of the film covering diamond (111) facet. The face centred cubic γ-(Fe,Ni) and the orthorhombic Fe3C phases are found coexisting on the film surface. Particles in tetragonal shape were found to be (001) facet of γ-(Fe,Ni). It is suggested that graphite cannot be directly converted into diamond in molten state at high pressure and high temperature, and the γ-(Fe,Ni) phase plays an important role as a catalyst in the formation of diamond.