Surface modification of γ-Fe₂O₃ nanoparticles synthesized by a chemically induced transition has been attempted by adding ZnCl₂ during synthesis. The structure of the modified particles was studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results showed that ZnFe₂O₄ grew epitaxially on the γ- Fe₂O₃ crystallites to form composite nanoparticles with the spinel structure, on which FeCl₃ might be adsorbed. The apparent grain size d_c estimated by XRD using the Scherrer equation was larger rather than smaller than the average particle size measured by TEM. This paradox can be explained by the effect of absorption in the coating heterolayer.