Co68.25Fe4.5Si12.25B15 naked amorphous wires of 60μm in diameter are prepared by melt extraction technology and annealed by DC-Joule heat at current from 40mA to 250mA for 10min. Impedance response is measured at low frequencies(≤13MHz) using HP4192 impedance analyzer. We get better GMI effect by heating wires at 60mA and 180mA.The connection between GMI response and transformation of structure under different heat current has been discussed. After proper heat treatment, magneto impedance response is strong even at 0.5MHz with GMI amplitude of 193.1% and field sensitivity of 59.4%/Oe. The maximum △Z/Z reaches 476.9% and the largest field sensitivity is up to 91.8%/Oe. These results indicate proper annealed Co-based melt extraction wires are suitable for high sensitivity sensor applications.