To concentrate sea water, it is necessary to develop a high pressure reverse osmosis system. First, the effect of applied pressure and feed concentration on flux and rejection was measured using a commercially available membrane. Secondly, the experimental data were analyzed by Spiegler-Kedem equations. The equations are also confirmed to be effective in high pressure reverse osmosis system. The membrane shows high rejection under applied pressure of 20 MPa with feed solute concentration of 12 wt% NaCl, insuring a good stability of the dense layer to pressure and to feed concentration. On the other hand, the support layer is compacted under 20 MPa, which marks the onset of the resistance to permeation.