Membrane fouling has been associated with the size of solutes, i.e., macromolecules and colloids in membrane filtration. Colloidal fouling propensity was investigated in terms of transport characteristics by the thermodynamic transport model combined with concentration polarization and flow field-flow fractionation (flow FFF). Transport characteristics in membranes were focused on, especially nano-colloid fouling in UF membranes. Whey protein and 30 nm of latex microsphere suspensions were used as test solutions. Transport parameters were determined in terms of mass transfer coefficient (k), solute permeability (P-m), and reflection coefficient (sigma) by non-linear estimation. The flow FFF provided a consistent explanation for the nano-colloid transport, depending on the physicochemical interactions between membrane and solute through the transport parameters determined from filtration tests. The second and third moments, variance and skewness were expected to be very useful quantitative information on the membrane fouling propensity of a certain solute.