This report focuses on ac magnetic properties of (Fe 0.5 Co 0.5 ) 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy in the high temperature. The effects of replacing 50% Fe by Co in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy on the static magnetic properties have also been studied. Addition of Co leads to an increase around 110K of the Curie temperature of the amorphous phase, much higher than Ge-containing Finemet alloy. The material has much better ac magnetic properties than Hitperm alloy. An excellent stability of initial permeability in high temperature (up to 650K) and high frequency (up to 5×10 5 Hz) can be observed with respect to the Co-free alloy.