Magnetorheological (MR) fluids based on glycol, iron powder, polyvinylpyrrolidone (PVP), and carbon nanotubes (CNTs) were prepared. Effects of polyvinylpyrrolidone and carbon nanotubes on sedimentation stability and magnetorheological properties were studied. It is found that the synergetic effects of PVP and CNTs improve the sedimentation stability significantly, and the addition of CNTs reduces the sedimentation velocity and increases the equilibrium sedimentation ratio of the magnetizable particles in MR fluids remarkably. The addition of PVP can reduce the sedimentation velocity of the magnetizable particles, but cannot increase the equilibrium sedimentation ratio and will not change the up trend of apparent viscosity with the increasing intensity of the external magnetic field. When the PVP content is lower, the increment of original apparent viscosity of the MR fluids at zero-intensity of magnetic field is inconspicuous, and their values of apparent viscosity under magnetic field are similar. However, the apparent viscosity of the MR fluids increases tremendously when the contents of PVP increase to certain degree. The results show that the synergetic effects of PVP and CNTs not only improve the sedimentation stability of the MR fluid but also promote its magnetorheological effect. (c) 2006 Wiley Periodicals, Inc.