In this work, Cu-Co-Nb magnetic composite alloys were obtained by hot-pressing under argon atmosphere of an alloy prepared by mechanical mixing. For the preparation of this powder master alloy, it was mixed 15wt% of intermetallic Nb3Sn, synthesized by mechanical alloying of elemental Nb and Sn and subsequent thermal treatment, and a Cu-10wt%Co solid solution, also prepared by mechanical alloying of elemental Cu and Co. A laboratory mill working at 250 rpm under argon atmosphere during 5 and 10h was used to disperse ultrafine Nb3Sn particles into the copper-10wt%Co matrix. Microstructure of resulting powders and consolidated materials were characterized by means of X-ray diffraction, scanning electron microscopy, and electron micro probe analysis. Magnetic properties and mechanical properties were measured by means of a vibrating sample magnetometer and a durometer, respectively. Fitting of the X-ray diffraction patterns with the Rietveld method, revealed that during the hot pressing process, the Nb3Sn particles reacted with cobalt to form (Co,Sn)2Nb, liberating some Sn that difussed in Cu forming a Cu-Sn solid solution. The influence of the milling time on this structural change and the associated variation of magnetic properties are discussed.