Objective To investigate the mechanism of γ-Fe2O3 / Ni2O3 composite nanoparticle stably dispersed in the ferrofluids after Fe(NO3)3 treatment. Methods The FeOOH/ Ni(OH)2 precursor was prepared by coprecipitation, and was then treated with the FeCl2 solution to obtain composite nanoparticles containing γ-Fe2O3 core, Ni2O3 external shell and FeCl3·6H2O outermost layer. The nanoparticles were then treated with Fe(NO3)3 to obtain modified particles with stable surface properties for the preparation of ionic ferrofluids. The magneticity, morphology, crystal structure, chemical composition and structure of the composite nanoparticles after Fe( NO3 ) 3 treatment were analyzed by VSM, TEM, XRD, EDX and XPS. Results The particles after treatment were slightly larger (about 11 nm), but the major component γ-Fe2 O3 located in the core of particles remained unchanged. And the surface of the particles was covered with a Fe(NO3 ) 3 ·9H2 O corrosion-resistant layer. Conclusion The corrosion-resistant layer covering the particles after Fe(NO3 ) 3 treatment and the electrostatic repulsion formed by the same species H+ or OH- adsorbed onto the surface of the particles during the preparation of magnetic liquid made the particles stably dispersed in the ferrofluids.