Abstract: Structure of the newly developed hydro-viscous soft start transmission is introduced and the hydroviscous-braking model is presented. The study on the hydro-viscous brake shows that a coupling relationship exists among the parameters of oil film thickness, hydro-viscous torque and friction disc speed. The variation of hydro-viscous torque is determined by the trend speeds to zero offriction disc speed and oil film thickness. When different patterns of oil film thickness are applied, hydro-viscous torque and friction disc speed will have obviously distinct characteristics.