In this study, monodisperse Fe ₃ O ₄ nanoparticles were synthesized successfully using a sonochemical method in the presence of 3-aminopropyltriethoxysilane (APTES). The morphology, microstructure and magnetic properties of the bare Fe ₃ O ₄ and APTES-coated Fe ₃ O ₄ were investigated in detail by TEM, XRD, FTIR and SQUID. It was found that APTES-coated Fe ₃ O ₄ showed relatively good dispersion with a narrow size distribution of 8.4 ± 2.1 nm diameter. The functionalization of Fe ₃ O ₄ was proved to be covalent linking between Fe ₃ O ₄ and APTES. The field-dependent magnetization curve indicated superparamagnetic behavior of Fe ₃ O ₄ -APTES with a saturation magnetization ( M _s ) of 70.5 emu g ⁻¹ at room temperature. A magnetorheological (MR) fluid was prepared using the obtained Fe ₃ O ₄ -APTES nanoparticles with 25 wt% particles, and its MR properties were tested using a Physica MCR301 rheometer fitted with an MRmodule. The results showed that the as-prepared APTES-coated Fe ₃ O ₄ nanoparticle-based MR fluid exhibited typical MR effects, with increasing viscosity, shear stress and yield stress depending on the applied magnetic field strength.