Monodisperse 2 nm Fe100-xPtx nanoparticles were synthesized using polyol process. As-synthesized FePt (x = 37) and FePt3 (x = 66) nanoparticles have the chemically-disordered face-centred cubic (fcc) structure. To achieve ordered structure (L1(2) phase), high-temperature annealing is required. During annealing, coalescence occurs and large agglomeration is formed with multi-domain particles. Core/shell structure, CoFe2O4 as a shell, is prepared to prevent the uncontrolled agglomeration. The shell thickness of CoFe2O4 is changed and controlled by varying ratio of initial precursor. The results show a decrease in coercivity by increasing shell thickness. This reveals effective exchange coupling between core and shell and confirms that shell thickness has a major role in magnetic properties of FePt3/CoFe2O4 nanoparticles. (C) 2011 Elsevier B. V. All rights reserved.