Numerical and experimental analyses were performed for the RO desalination system using a static pressure head, during which two cases of the system channel configuration were considered. One case was where only one side of the annular channel was laminated with the RO membrane, and the other was where both sides of the annular channel were laminated with the RO membrane, In both cases numerical results were obtained for the effects of the width and the axial length of the channel, the submerged depth, the pure water permeability and the solute permeability of membrane, and the physical properties of seawater or brackish water on the axial variation of velocity, concentration, and pressure in laminar natural convection flow, caused by the concentration difference in the moderately narrow channel, The effect of these parameters on the axial variation of the flow rate of transmitted fresh water was also examined. From the numerical results the dimensionless expressions were formulated to predict the flow rate of transmitted fresh water from the parameters, The field experiments on the desalination system of seawater using a static pressure head due to the depth of the sea in the former case were carried out in the East China Sea and the Sea of Japan, Experimental results of the flow rate of transmitted fresh water permeated through the membrane were found to be in good agreement with the expression, and the applicability and the usefulness of the expression were confirmed.