Organic thermoelectric materials, which can transform heat flow into electricity, have great potential for flexible, ultra-low-cost and large-area thermoelectric applications. Despite rapid developments of organic thermoelectric materials, exploration and investigation of promising organic thermoelectric semiconductors still remain as a challenge. Here, the thermoelectric properties of several p- and n-type organic semiconductors are investigated and studied, in particular, how the electric field modulations of the Seebeck coefficient in organic field-effect transistors (OFETs) compare with the Seebeck coefficient in chemically doped films. The extracted relationship between the Seebeck coefficient (S) and electrical conductivity (sigma) from the field-effect transistor (FET) geometry is in good agreement with that of chemically doped films, enabling the investigation of the trade-off relationship among sigma, S, carrier concentration, and charging level. The results make OFETs an effective candidate for the thermoelectric studies of organic semiconductors.