Magneteroheogical Fluids have very wide application due to the particular magneteroheogical effect. It is the key to develop ment of magneteroheological fluids and application to research on the magneteroheological effect. This paper discusses the magnetorheological effect for Magnetorheological (MR) fluids from the view of theory and experiment. It describes the influence of the applied magnetic field, volume fraction of the particles and the size of the magnetic particles on the rheological effect of MR fluids. It is shown that the experiment result agrees with the theoretic one. That provide the basis for the engineering development and the engineering applications.