Second harmonic amplitude (V2f) of Giant Magneto-Impedance (GMI) voltage of (Fe6Co94)72.5Si12.5B15 amorphous wires has been studied systematically for as-cast and Joule-heated states. Joule heating was carried out at current densities (J) of 20-40 A/mm2 for a period of 1-300 min. The amplitude of the second harmonic voltage (V2f) initially increased with J, attained maximum and then decreased. The maximum amplitude of V2f was observed for the sample annealed at J=25 A/mm2 for 5 min. The second harmonic peak shifted towards the higher field side with the increase of annealing current density and annealing period (tp). Few-fold increase in the amplitude of the second harmonic voltage (V2f) was observed due to the presence of a small dc bias current (Ib) of amplitude 1 mA. The asymmetry in V2f was also observed in a Joule-heated sample. The observed asymmetry has been correlated with the bias field generated between the surface crystallized layer and the amorphous core. A small bias current of amplitude of 1 mA was applied along the direction of the applied field to minimize the asymmetry. © 2007 Elsevier B.V. All rights reserved.