The Ilan Plain is identified as the western extension of the Okinawa Trough in the northeastern Taiwan subduction system and is considered one of the most active geothermal areas in Taiwan. An east-west extending magma-like body caused by the westward of the Okinawa Trough had been revealed from seismic and geomagnetic studies. Two geological models have been suggested as an extensional strike-slip fault and normal fault models for the deep geothermal field in NE Taiwan. The difference of the main fault in the two geology models is the active normal strike-slip fault or inactive thrust fault and its dip directions. We investigated the electrical resistivity structure beneath the Ilan Plain using audio-magnetotelluric data to delineate the location of its geothermal reservoirs. The 3D electrical resistivity model is highly correlated with the normal fault model. The electrical resistivity model shows that the high conductivity anomaly distributes in the depths of 500-1,100 m south of the drilled geothermal observation wells. We correlate the anomalous high conductor with the reservoir and it could be controlled by the Chuoshui Fault and the Chukeng Fault. The meteoric water may propagate down through the high-permeability fault system that creates the geothermal system.