In this paper, we study the device performance of the graphene nanoribbon tunnel field effect transistors in the sub 0.5V bias voltage range by non-equilibrium Green's function calculation. The ON/OFF ratio is found to increases with decreasing bias voltage, as the improved current suppression in the OFF state at low bias is more effective than the reduction of carriers in the ON state. Abrupt increase of the subthreshold slope (SS) from sub-thermal ∼30 mV/dec to 60 mV/dec below V Bias of 0.1 V is observed. SS relates to the occupancy of carriers at the threshold energy of band-to-band tunneling. We furthermore discuss the constant potential barrier scaling of the graphene channel width and length.