Fe 3 O 4 nanoparticle/organic hybrids were synthesized via hydrolysis using iron (III) acetylacetonate at ∼80°C. The synthesis of Fe 3 O 4 was confirmed by X-ray diffraction, selected-area diffraction, and X-ray photoelectron spectroscopy. Fe 3 O 4 nanoparticles in the organic matrix had diameters ranging from 7 to 13nm depending on the conditions of hydrolysis. The saturation magnetization of the hybrid increased with an increase in the particle size. When the hybrid contained Fe 3 O 4 particles with a size of less than 10nm, it exhibited superparamagnetic behavior. The blocking temperature of the hybrid containing Fe 3 O 4 particles with a size of 7.3nm was 200K, and it increased to 310K as the particle size increased to 9.1nm. A hybrid containing Fe 3 O 4 particles of size greater than 10nm was ferrimagnetic, and underwent Verwey transition at 130K. Under a magnetic field, a suspension of the hybrid in silicone oil revealed the magnetorheological effect. The yield stress of the fluid was dependent on the saturation magnetization of Fe 3 O 4 nanoparticles in the hybrid, the strength of the magnetic field, and the amount of the hybrid.