Abstract A novel magnetorheological fluid, in which the surface of iron particles is coated with poly(butyl acrylate) by surface-initiated atom transfer radical polymerization (ATRP), is investigated. The polymer coating procedure includes two steps, which are immobilization of initiator: 2-(4-chlorosulfonylphenyl)-ethyltrichlorosilane (CTCS) on the iron particles' surface and graft polymerization of butyl acrylate from the surface. The surface coating is characterized by FTIR and SEM. This magnetorheological fluid has controllable off-state viscosity and high shear yield stress. Coating polymer on the iron particles' surface by ATRP can significantly reduce iron particles' settling and improve stability of the MR fluid. Glass transition temperature is obtained using the step-scan DSC method. The molecular weight and conversion can be controlled by the molar ratio of monomer to initiator, reaction temperature and time. The reaction is first order determined by the plot of ln[M]0/[M] against polymerization time.