The carrier transport mechanism through the p-diamond/metal interface was studied by measuring specific contact resistances (ρc) using a transmission line method for Ti, Mo, and Cr (carbide forming metals) and Pd and Co (carbon soluble metals) metals contacting to the boron-doped polycrystalline diamond films. Schottky barrier heights (ΦB) of around 0.5 eV were measured for the annealed contacts. The present result indicates that formation of thermally stable graphite layers at the diamond/metal interfaces upon annealing would pin the Fermi level of the p-diamond. This model led to the preparation of in situ Ohmic contacts by depositing a thin diamondlike carbon on the p-diamond surface prior to metal deposition, and also to excellent Schottky contacts with breakdown voltages higher than 900 V. The present experiment concluded that the existence of a graphite layer at the diamond/metal interface controlled the electrical properties through the p-diamond/metal interface. © 1997 American Institute of Physics.