As a new elemental two-dimensional material, layered black phosphorus (BP) has revealed its intriguing properties and is considered to be a potential thermoelectric material. However, the ambiguous carrier thermoelectric transport in layered BP limits its development. In this work, based on variable-range hopping transport and percolation theory, we have developed a unified thermoelectric-transport model to investigate the Seebeck effect of BP field-effect transistors (FETs). The proposed model can characterize the effect of temperature, disorder, and the carrier concentration on carrier thermoelectric transport. The simulation results show abnormal temperature-dependence with decreasing disorder degree in BP FETs. The proposed model will be helpful to understand the thermoelectric transport properties of layered BP and design desired thermoelectric devices. (C) 2017 Elsevier B.V. All rights reserved.