The electrical characteristics of phenylene-based molecular devices were assessed. The device consisted of nanogap electrodes and phenylenebased conjugated molecules. One nanogap electrode was obtained by the electromigration of a Au nanowire modified with a self-assembled monolayer (SAM) of 4,4-p-terphenyldithiol (TPDT). The other nanogap electrode was fabricated by the shadow evaporation of metals and subsequent deposition of SAM. Some devices obtained by the electromigration of Au nanowire modified with SAMs exhibited a large activation energy of electrical conduction of up to 0.26 eV. This large activation energy coincides with the intramolecular barrier estimated by ab initio molecular orbital calculations. On the other hand, the devices composed of shadow evaporated nanogaps and subsequently deposited TPDT exhibited a comparatively small activation energy. Neither device showed a clear gate effect with an electrical field up to 3 MV/cm. These results indicate that the electrical characteristics of molecular devices are affected by the fabrication process and the resultant molecule-electrode configuration.