To realize the application of forward osmosis (FO) membranes to industrial-level water treatment, the correct selection of the draw solute (DS) and good membrane performance is necessary. In this study, the FO flux behavior was investigated using draw solutes of inorganic salt (NaCl) and neutral polymers (PEGs) with molecular weights of 200-8300. The FO flux dramatically decreased with increasing molecular weight of the DS. Severe internal concentration polarization originated from the lower diffusivity of the larger DS molecules, which brought about the flux reduction. The experimental data were analyzed with a model developed using a virial expansion equation for osmotic pressure in the case of the polymeric draw solutions. The analyzed results fit well with the experimental data, which allowed us to predict a relation between FO performance and the molecular weights of the draw solutes for different FO membranes.