Recently, nanocrystalline Fe-Si-B-P-Cu soft magnetic alloys, NANOMET® with high Fe contents of above 83 at% have attracted much attention as a future core material for high performance magnetic applications because of their high magnetic flux density (Bs ∼ 1.80 T), low coercivity (Hc), significantly low core loss (W) and low materials cost -. Small addition of Co (∼ 4 to 5 at%) is shown to be effective in producing wide ribbons for commercial applications (such as transformers, motors etc.). High concentrations of Fe and Co (very close to the limit for the formation of amorphous state) in the alloys were used to obtain high Bs (∼ 1.84 T) . Simultaneous existence of Bs similar to oriented steel and Hc lower than it are the driving factors to further increase in magnetic elements in the alloy. Generally it is believed that the amorphous structure (a broad halo in X-ray diffraction curve) of as quenched ribbons is very important for achieving a uniform nano-crystalline structure with low coercivity (Hc) after optimum heat treatment. We have noticed that the nanocrystalline Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy shows low Hc < 10 A/m, but a minor increase in Co from 4 to 5 at.% (Fe81.3Co5Si0.5B9.5P4Cu0.7) results in a drastic increase in Hc to ∼ 60 A/m. In terms of structure both the alloys in as quenched state exhibit similar X-ray diffraction patterns (i.e. X-ray amorphous). The B of this alloy approaches to ∼ 1.9 T.