This study examined the effect of concentration polarization on permeate flux in forward osmosis membrane process for saline water. The forward osmosis membranes used for experiments are commercialized cellulosic membrane(HTI-CAT, Albany, OR). There is a deviation in the permeate flux with the increasing salt concentration. This nonlinearity of permeate flux was due to the reduction in effective osmotic pressure caused by dilutive concentration polarization(DCP) phenomena. The concentration polarization(CP) formation and the corresponding variation in permeate flux behavior varied greatly depending on membrane orientation as well as draw solution concentration and cross-flow velocity. The CP increased with increasing the draw solution`s concentration and showed nonlinear profiles regardless of concentration. And the nonlinearlity is more pronounced in the mode of draw solution facing support layer(DS-SL) than in the mode of draw solution facing active layer(DS-AL). The behavior of the former is called internal concentration polarization(ICP) and mainly caused by the porous internal structure of an asymmetric membrane. The water permeability coefficient was 1.8081x10-7 for DS-AL and 1.0957x10-7 m/s atm for DS-SL mode. The corresponding membrane resistance was 5.5306x106 and 9.1266x106 s atm/m, respectively. The fraction of CP resistance in the total permeation resistance was 0.28~0.72 for DS-AL mode and 0.33~0.86 for DS-SL mode. The formation and dissipation of CP in the draw solution due to the permeate water reveals different oscillating behavior according to the membrane orientation.