Magnetorheological suspensions (MRS) based on mixtures of two commercial carbonyl iron powders (BASF grades CL and SU) as magnetic phase and hydrocarbon oil as liquid phase were prepared. CL and SU are both soft magnetic powders, but CL is a coarse powder, while SU is a fine one. The total mass fraction of iron was 80% w/w each formulation. Hydrophilic fumed silica (5% w/w of Aerosil® 200) was used to reduce the settling. The mixing ratios were: CL 0%, CL 20%, CL 40%, CL 60%, CL 80% and CL 100%. A MRS, the mixture CL 80%, showed considerable reduction of the plastic viscosity without field, in the range of 100-500 s -1, when compared to the MRS with just one powder. The yield stress values under applied field H ∼ 340 kA/m were: 18.1 kPa for the MRS CL 0%, 18.3 kPa for the MRS's CL 20% and CL 40%, 20.0 kPa for the MRS CL 60%, 22.3 kPa for the MRS CL 80% and 23.3 kPa for the MRS CL 100%, respectively. For comparison, a sample of commercial MRF-132LD (Lord Corp.) in the same conditions showed yield value of 21.2 ± 0.6 kPa. On the other hand, another MRS, CL 60%, showed an increment of ∼ 33% on the normal force, with relation to the MRS prepared with just CL or just SU powders, above 150 kA/m. Therefore, mixing carbonyl iron powders with different particle sizes can improve the performance of MRS, decreasing the 'off plastic viscosity, and increasing the MR effect. © World Scientific Publishing Company.197-913321338Farris, R.J., Prediction of the Viscosity of Multimodal Suspensions from Unimodal Viscosity Data (1968) Trans. Soc. Rheol., 12 (2), pp. 281-301He, D., Ekere, N.N., Viscosity of concentrated noncolloidal bidisperse suspensions (2001) Rheol. Acta, 40, pp. 591-598Ota, M., Miyamoto, T., Optimum particle size distribution of an electrorheological fluid (1994) J. Appl. Phys., 76 (9), pp. 5528-5532Wang, Z.W., Lin, Z.F., Tao, R.B., Influence of the size distribution on the viscous property of an electrorheological fluid (1997) Chin. Phys. Lett., 14 (2), pp. 151-154Tan, Z.J., Zou, X.W., Zhang, W.B., Jin, Z.Z., Influences of the size and dielectric properties of particles on electrorheological response (1999) Phys. Rev. E, 59 (3), pp. 3177-3181Foister, R.T., Magnetorheological fluids, US Patent 5,667,715 - Sep. 16, 1997Weiss, K.D., Carlson, J.D., Nixon, D.A., Method and MRF formulations for increasing the output of a MRF, US Patent 6,027,664 - Feb. 22, 2000Lim, S.T., Cho, M.S., Jang, I.B., Choi, H.J., (2004) J. Mag. Mag. Mat., 282, pp. 170-173Mezger, T.G., (2002) The Rheology Handbook, pp. 33-40. , Vincentz Verlag, HannoverWollny, K., Läuger, J., Huck, S., Magneto-sweep - A new method for characterizing the viscoelastic properties of MR fluids (2002) Applied Rheol., 12 (1), pp. 25-31Lemaire, E., Meunier, A., Bossis, G., Liu, J., Felt, D., Bashtovoi, P., Matoussevitch, N., Influence of the particle size on the rheology of magnetorheological fluids (1995) J. Rheol., 39 (5), pp. 1011-1020Wu, C.W., Conrad, H., Influence of mixed particle size on electrorheological response (1998) J. Appl. Phys., 83 (7), pp. 3880-3884See, H., Kawai, A., Ikazaki, F., The effect of mixing particles of different size on the electrorheological response under steady shear flow (2002) Rheol. Acta, 41, pp. 55-60