Fe-Si-B amorphous alloys with less than 80 at% Fe are now in practical use due to their excellent magnetic softness (low coercivity Hc) combined with rather high saturation magnetic polarization (Js) which basically owing to the lack of intrinsic magnetic anisotropy and the high Fe content, respectively. In order to obtain high Js, high Fe content is required. However, alloys with high Fe content exceeding the limit usually have the as-quenched structure consisting of coarse α-Fe grains in the amorphous matrix, which results in inferior magnetic softness. We have developed a new Fe85.2B10P4Cu0.8 nanocrystalline soft magnetic alloy ribbon (with 5 mm in width and about 20 μm in thickness) made from industrial raw materials in air atmosphere. The as-quenched structure of Fe85.2B10P4Cu 0.8 alloy has heterogeneous amorphous structure (a large amount of extremely small α-Fe clusters in addition to amorphous phase). Homogeneous nanocrystalline structure composed of α-Fe grains with a size ∼19 nm was realized by crystallizing the hetero-amorphous alloy. The nanocrystalline alloy exhibit high Js ∼ 1.83 T (comparable to the commercial Fe-3.5 mass% Si steel) and extremely low Hc ∼ 6.0 A/m. Additionally the alloy has a large economical and industrial advantage of lower material cost and good reproductivity, which has a high potential for the power applications.