A ferrite core of the invention is useful for reducing the power loss of Ni--Cu--Zn ferrites, and so for particle accelerators, and power transformers. This core comprises a ferrite magnetic material containing as major components 47 to 50 mol % of iron oxide calculated as Fe.sub.2 O.sub.3, 10 to 25 mol % of nickel oxide calculated as NiO, 2 to 15 mol % of copper oxide calculated as CuO and 15 to 35 mol % of zinc oxide calculated as ZnO, and further containing as subordinate components 0.05 to 1.5 wt % of cobalt oxide calculated as CoO, 0.05 to 0.8 wt % of tungsten oxide calculated as WO.sub.3 and 0.03 to 0.5 wt % of bismuth oxide calculated as Bi.sub.2 O.sub.3, all based on the major components. Consequently, there is achieved a ferrite core having a power loss at 100.degree. C. of up to 210 kW/m.sup.3, and a power loss at 25.degree. C. of up to 140 kW/m.sup.3, as measured at f.multidot.Bm product=25 kTHz (f=1 to 10 MHz).