We synthesized Fe3O4/polystyrene (PS) composite nanoparticles via a Shirasu porous glass (SPG) membrane emulsification technique and examined their magnetorheological (MR) properties under external magnetic field strengths. The SPG method was adopted for achieving a narrow size distribution of the product nanoparticles containing magnetite. The structure and morphology of the samples were analyzed by transmission electron microscopy and their thermal characteristics were evaluated by thermogravimetric analysis. The MR behavior of a Fe3O4/PS composite-based MR fluid dispersed in silicone oil was examined by a rotational rheometer under an extra magnetic field strength. The experimental results obtained from both the steady shear and dynamic oscillation tests showed that the fluid exhibited a typical MR behavior under an applied magnetic field. (C) 2017 Elsevier Ltd. All rights reserved.