Abstract: A new phase, i, e. hexagonal cobalt carbide with a=0.268nm and c = 0.433nm was formed by carbon ion implantation. The only phase formed for the three ferromagnetic metals, i. e. bcc. Fe, hcp. Co and fcc. Ni after carbon ion implantation was the corresponding hexagonal carbide respectively. Structural simplicity was concluded to be the dominant characteristic of phase formation by ion implantation comparing to the conventional thermal process. Amorphous alloy may also be readily formed in ion beam process. The conclusions may be used to direct experiments in general and expected other new phase to be found.