The effect of concentration polarization on performances of integrally asymmetric and thin film composite forward osmosis (FO) membranes was systematically investigated in two membrane configurations — active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS). Water fluxes of the membranes using pure water as FS in AL-DS were 1.7 to 2.6 times higher than those in AL-FS. However, with increasing FS concentration, the water flux in AL-DS declined rapidly and became lower than that in AL-FS. A modeling study showed that the reversal of water flux in the two configurations were resulted from concentrative ECP, and theoretical FS concentrations at which water fluxes in both configurations were equal were inversely proportional to the initial water flux of the membranes. The turnover of the water flux in AL-DS and AL-FS was also observed within 8 h when seawater and river water were used as DS and FS, respectively. Moreover, the AL-DS faced the challenge of high fouling propensity and difficulty in cleaning more than the AL-FS. This study suggests that AL-FS membrane configuration is proper for both PRO and FO processes employing real natural water in terms of high membrane performance and ease of cleaning.