The various aspects of a nanoscopic magnetic field sensor, which was based on extraordinary magnetoresistance (EMR), were discussed. EMR devices were fabricated by the electron beam lithographic methods. EMR devices could be used as read-head sensors for ultrahigh-density magnetic recording, magnetic imaging, and field sensing biological nanostructures. The EMR values at room temperature was increased by 35% at an applied field of 0.05 T due to the sensor with an active volume of 35nm length×30 nm width×20 nm height.