Co1-xNix alloy nanoparticles (x = 0.2, 0.5, 0.6, and 0.8) with the diameter 15-28 nm attached on the surface of multi-walled carbon nanotubes (MWCNTs) were prepared to form Co1-xNix/MWCNT nanocomposites by microwave irradiation. Experimental results demonstrated that Co1-xNix alloy nanoparticles with quasi-spherical and face-centered cubic structure had been attached on the MWCNTs, the composition and size of Co1-xNix alloy nanoparticles could be controlled through adjusting the atomic ratios of metal Co to Ni in the mixed acetate solution, the microwave power and microwave irradiation time, respectively. Both the coercivity and the saturation magnetization of Co1-xNix alloy nanoparticles increased with increasing Co concentration from x = 0.8 to 0.5, and decreased when Co concentration was increased from x = 0.5 to 0.2. These confirm that microwave synthesis is promising for fabricating alloy nanoparticles attached on MWCNTs for magnetic storage and ultra high-density magnetic recording applications. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.