The hydraulic pressure plays an important role in the osmotically driven membrane processes. It is demonstrated in this study that the hydraulic pressures applied on the feed solution and draw solution are independent from each other due to the existence of the higher osmotic pressure of the draw solution. While the hydraulic pressure on the draw solution affects water flux marginally by modifying the osmotic pressure of the draw solution, the hydraulic pressure on the feed solution drives water across the membrane much more effectively, with a permeability similar to that in reverse osmosis and pressure assisted osmosis processes. Based on different controlling mechanisms of water transport under osmotic pressure and hydraulic pressure, a mathematical model was developed for water flux in the osmotically driven membrane process with simultaneous applications of hydraulic pressures on both sides of the membrane. The independence of hydraulic pressures on the opposite sides of the membrane and the validity of the developed mathematical model were verified experimentally.