AbstractThe highly filled anisotropic polyurethane (PU) magnetorheological elastomers (MREs) were prepared through an in‐situ one‐step polycondensation process under a magnetic field. The carbonyl iron formed chain‐like structure, which was fixed in the PU matrix. The plasticizer diisooctyl phthalate (DOP) was incorporated into PU to soften the matrix and improve the MR effect. The influence of DOP on the microstructure and properties of PU MREs were investigated. The incorporation of DOP reduced the viscosity of the prepolymer and made the carbonyl iron align more easily in the PU matrix. The aligned chain‐like structure of carbonyl iron in PU greatly enhanced the thermal conductivity and the compressive properties of PU MREs. The incorporation of DOP reduced the modulus of PU MREs and the glass transition temperature of the soft segments of PU. But highly filled carbonyl iron and DOP led to a decrease in the thermal stability to some extent. The MR test showed that DOP plasticization significantly enhanced both absolute and relative MR effect simultaneously. With 70 wt% carbonyl iron and 15 wt% DOP (the weight ratio of Fe: PU: DOP is 70 : 15: 15), the absolute and relative MR effects of anisotropic PU MREs were ∼ 1.16 MPa and ∼ 386.7%, ∼ 3.5 and ∼ 58 times of the PU MRE without the plasticizer at the same iron content. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012