The structure, phase composition, dispersion and magnetic properties of nanopowders containing blends of copper with iron and cobalt obtained by mechanical milling in ultrasonic ball mill (ultrasonic milling) under applied magnetic field are studied by means of the x-ray analysis, electron microscopy, Mössbauer spectroscopy methods and magnetometry. As shown, during ultrasonic milling, the next phases are appeared: one crystalline f.c.c. phase is formed in the Cu + Co powder blends (PB)—the oversaturated copper–cobalt solid solution; two crystalline f.c.c. and b.c.c. phases in the Cu + Fe PB are formed, $i.e.$ oversaturated solid solutions of iron in copper or of copper in iron. Ball milling in ultrasonic ball mill leads to essential structure refinement up to nanometres’ sizes (up to 40 nm) and to increasing in dislocation density up to the level of 10$^{12}$–10$^{13}$ cm$^{-2}$. Increasing of the treatment time over 20 hours leads to saturation of PB with oxygen and to formation of considerable quantity of oxides that makes for structure dispersion but decreases threefold the saturation magnetization of Cu–Fe nanopowders, while in essence, does not impact on the magnetization of Cu–Co nanopowders.