Magnetorheological fluid (MR fluid), a kind of smart material, has been used as a new pressure-carrying medium in magnetorheological pressure forming (MRPF). The mechanical property of MR fluid under the pressure significantly affects the sheet formability. However, there is little knowledge on the deformation behavior of MR fluid under three-dimensional stress states. In this paper, a new procedure via a combination of extrusion test and FEM simulation has been proposed to determine the flow stress of MR fluids. The experimental device for extrusion test of MR fluids was designed. The flow stresses of a MR fluid (MRF-J01T) under four different magnetic fields were determined through the proposed procedure. In addition, the obtained flow stresses were used in the following FEM simulations to verify the accuracy by comparing with the experimental results. The simulation results were in good agreement with the experimental data, which supports the correctness and practicability of the proposed method. (C) 2016 Elsevier B.V. All rights reserved.