The aim of this work is the study of the milling conditions (milling temperature T and milling intensity I) and the influence of post-milling heat treatments on the microstructure of the Cu-80(Fe0.3Co0.7)(20) compound, the giant magnetoresistance (GMR) properties of which strongly depend on the size of the magnetic particles and their spatial distribution. Batches of Cu88Fe12 and Cu72Co28 powders were prepared by mechanical alloying (MA) elemental powders at T = 300 K and I = 1000 m/s(2). The ternary compound was produced by ball milling a mixture of these two binary compounds in the milling temperature range 300 K - 473 K with a milling intensity ranging from 1000 m/s(2) to 4500 m/s(2). Annealing was then performed under vacuum at 773 K over a time range of 15 min - 24 h. The compounds were characterised by neutron powder diffraction, Small Angle Neutron Scattering experiments and magnetic measurements. The results showed that the majority of the Fe and Co atoms are probably in f.c.c. Fe-Co superparamagnetic particles after milling. The remaining atoms seem to be in solid solution in the Cu-rich matrix or in very small superparamagnetic germs. After annealing, the particles coarsen and transform partly to b.c.c. phase. Their ferromagnetic volume fraction increases with the annealing time. Effects of the milling conditions have been proved.