Diamond nucleation and growth mechanisms on nickel substrates were investigated by ellipsometric monitoring, X-ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. It was found that carbonization, diamond nucleation, and graphite growth stages exist and that the change in growth phase from diamond to graphite is earlier for a process with biasing than without biasing due to the higher density of carbon and hydrocarbon radicals. It was also found that longer biasing treatment in high methane concentration preceding a growth process without biasing enhances the growth of hexagonal diamond on graphite.