Heterostructures from mechanically-assembled stacks of two-dimensional materials allow for versatile electronic de-vice applications. Here, we demonstrate the intrinsic charge transport behaviors in graphene-black phosphorus heterojunc-tion devices under different charge carrier densities and temperature regimes. At high carrier densities or in the ON state, tunneling through the Schottky barrier at the interface between graphene and black phosphorus dominates at low temper-atures. With temperature increasing, the Schottky barrier at the interface is vanishing, and the channel current starts to decrease with increasing temperature, behaving like a metal. While at low carrier densities or in the OFF state, thermal emission over the Schottky barrier at the interface dominates the carriers transport process. A barrier height of～67.3 meV can be extracted from the thermal emission-diffusion theory.