The formation of diamond and the behaviour of coexisting carbides with diamond in several 3d-transition metals (manganese, iron, cobalt, nickel)-graphite systems was studied under 7 GPa pressure up to 1700 ~ C from the view-point of diamond formation. In the nickel-graphite and cobalt-graphite systems, no stoichiometric carbide was formed, but MxC, which is thought to be the interstitial solid solution of carbon, was formed. In the iron-graphite system, on the other hand, the formation of two stoichiometric carbides, i.e. Fe3C and FeTCs, was found. In the manganese-graphite system, only one stoichiometric carbide Mn7Cs, which is isostructural with Fe7Ca, was formed. The diamond formation process in the presence of these transition metals is discussed in relation to the carbide formation.