Organic electrochemical transistors (OECTs) have been found to be excellent transducers for various types of biosensors. Here, we report highly sensitive epinephrine sensors based on OECTs prepared on glass substrates by a solution process. The device performance is optimized by immobilizing Nafion and carbon-based nanomaterials, including carbon nanotubes, graphene and graphene oxide, on the gate electrodes of OECTs. The detection limit of the sensors is down to 0.1 nM, which can cover the concentration level of epinephrine in medical detections. Considering that the devices can be prepared by a facile solution process with low cost, the highly sensitive epinephrine sensors will be ideal transducers for disposable applications in the future.