Specific contact resistance of Ti contact on phosphorus-doped diamond film has been characterized by means of circular type transfer length method. The resistance was significantly reduced down to ∼10 −3 Ω cm 2 orders using heavily phosphorus-doped diamond film (n + ) with phosphorus concentration over ∼10 20 cm −3 , although an ideal ohmic property was not obtained. The results are explained by a narrowing of the Schottky barrier width of Ti/n + interface for tunneling through the barrier to take place, indicating that the heavily doping is promising method for reducing the contact resistance for n-type diamond.