The study of Giant Magnetoimpedance (GMI) effect of the amorphous alloys system of Fe 72-x Co x Nb 6 B 10 Si 11 Cu 1 (for x = 35 and x = 40 at. percent Co) and AC magnetic susceptibility for the amorphous alloy of Fe 37 Co 35 Nb 6 B 11 Si 10 Cu 1 composition are presented in this chapter. The importance of GMI effect for the improvement of technological applications in sensor devices in amorphous magnetic Fe-and Co-based alloys is introduced; then it is described as the experimental procedure of magnetoimpedance and AC magnetic susceptibility measurements. The obtained results are discussed and finally the conclusions are presented.