Rectifying contacts to polycrystalline diamond (PCD) can he improved significantly by using a metal/undoped PCD/p-type doped PCD structure. In this paper, a model for the reverse current of a metal/undoped PCD/p-type doped PCD junction diode is proposed. In this model, the undoped PCD is treated as a slightly doped p-type semiconductor due to the existence of the deep level defects of acceptor-states. The electric field, which is needed to calculate the reverse current. is obtained by considering the p-type doped PCD layer and the undoped PCD layer as a high-low junction. The reverse current is therefore, modeled as the transport of holes through the undoped PCD layer based on the Poole-Frenkel effect. The simulated reverse current is compared with experimental results and a good agreement between the simulated and experimental results is achieved.