Monodisperse CoxNi1-xFe2O4 nanoparticles (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) with controllable composition attached on the multi-walled carbon nanotubes (MWCNTs) were prepared by microwavepolyol method. The composition of CoxNi1-xFe2O4 nanoparticles can be controlled through adjusting the atomic ratios of cobalt and nickel nitrate in the mixed solution. The influence of the microwave power and microwave irradiation time on the monodispersion of nanoparticles was also investigated. The results show quasi-spherical CoxNi1-xFe2O4 nanoparticles with the face-centered cubic structure and average crystallite size (6 nm) are uniformly dispersed on MWCNTs. The saturation magnetization of CoxNi1-xFe2O4/MWCNT nanocomposites increases gradually from 12.90 to 20.03 emu/g with increasing Co2+ concentration. The coercivity is almost zero at room temperature, which indicates the superparamagnetic behavior. (C) 2013 Published by Elsevier Ltd.