CoFe2-xSmxO4 (x=0-0.2) nanofibers with diameters about 100-300 nm have been prepared using the organic gel-thermal decomposition method. The composition, structure and magnetic properties of the CoFe2-xSmxO4 nanofibers were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, inductive coupling plasma mass analyzer and vibrating sample magnetometer. The CoFe2-xSmxO4 (x= 0-0.2) nanofibers obtained at 500-700 degrees C are of a single spinel structure. But, at 800 degrees C with a relatively high Sm content of 0.15-0.2 the spine] CoFe2-xSmxO4 ferrite is unstable and the second phase of perovskite SmFeO3 occurs. The crystalline grain sizes of the CoFe2-xSmxO4 nanofibers decrease with Sm contents, while increase with the calcination temperature. This grain reduction effect of the Sm3+ ions doping is largely owing to the lattice strain and stress induced by the substitution of Fe3+ ions with larger Sm3+ ions in the ferrite. The saturation magnetization and coercivity increase with the crystallite size in the range of 8.8-57.3 nm, while decrease with the Sm content from 0 to 0.2 owing to a smaller magnetic moment of Sm3+. ions. The perovskite SmFeO3 in the composite nanofibers may contribute to a high coercivity due to the interface pinning, lattice distortion and stress in the ferrite grain boundary fixing and hindering the domain wall motion. (C) 2011 Elsevier B.V. All rights reserved.