A new micro-mechanic-hydrodynamic model(MMH-Model) to predict the yield stress of magnetorheological fluids has been developed . The storage modulus of a MR fluid with a three-dimensional chain structure model and the apparent viscosity of a MR fluid using the balance between the magnetic torque and hydrodynamic torque are determined. The magnetic interaction between the particles and the wall, as well as the static coefficient of friction, are considered. The predictions of the model are compared to preliminary experimental data obtained in a controlled stress plate-plate rheometer and other theoretical predictions. It is found that the model gives almost the correct yield stress for the MRFs in the magnetic fields lower than the saturation field, but underestimates the experimental results obtained in the magnetic fields higher than the saturation field.