The measurements of the specific contact resistance (ρc) were carrie d out for Ti/Au ohmic contact to heavily boron-doped p-diamond (～1020cm-3) by the transmission line model (TLM).I-V measurements were per formed before and after annealing at 500℃,as well as in big current cases.The a nnealing effects on the ρc value were investigated.It is shown that heavy doping of the semiconductor and annealing are effective means to improve o hmic contacts.The changes of ρc value with the operating temperature are discussed.We suggest that the dominant transport mechanism at the metal/sem iconductor interface is tunneling.No light effects on the ρc value h ave been observed,the results show that diamond can be an ideal window material. In our experiments,the smallest ρc value of the Au/Ti/p-diamond reac hes ～10-4Ωcm2.