Disclosed is a Co-base magnetic alloy excellent in high-frequency magnetic properties, of which chemical composition is represented by the following general formula, by atomic %, (Co1-aFea)100-y-cM'yX'c, where M' is at least one element selected from the group consisting of V, Ti, Zr, Nb, Mo, Hf, Sc, Ta and W; X' is at least one element selected from the group consisting of Si and B; and a, y and c are defined by the formulas of a<0.35, 1.5<=y<=15, and 4<=c<=30, respectively. At least a part of the alloy structure of the alloy consists of crystal grains having an average grain size of not more than 50 nm. The alloy has a relative initial permeability of not more than 2000. It is suitably used for members of countermeasure against noise such as zero phase reactors for large current and electromagnetic shielding materials, inverter transformers, choke coils for active filters, antennas, smoothing choke coils, power supplies for lasers, pulse power magnetic members for accelerators.