Abstract: Carbon nanotube (CNT)/Fe3O4 composites were prepared by depositing Fe3O4 onto CNTs. The morphologies, magnetic properties, sedimentation stability and magnetorheological properties of the composite particles were investigated by transmission electron microscopy, vibrating sample magnetometer, and a viscosimeter with magnetic equipment. The diameter of CNT/Fe3O4 nanoparticles is about 60nm, while that of CNTs is about 20nm. The average density of the CNT/Fe3O4 nanoparticles is about 1.8g/cm3, which is only onefourth of the normal soft magnetic particles based on Fe3O4 or Fe powder. The CNT/Fe3O4 composites have a similar magnetic hysteresis loops to that of Fe3O4, indicating the good soft magnetic properties of the composite particles. The saturation induction density (Bs) of the composite particles is 0.21T，and the coercive force (Hc) is 7.67kA/m. Magnetorheological fluids based on such CNT/Fe3O4 composites show high sedimentation stability and good magnetorheological properties.