Based on the basic pore model and steric hindrance pore model, the extended pore model was founded to predict and optimize the nanofiltration processes for purifying fructo-oligosaccharides. In this model, the factors on which the rejection properties depended were summed up in pressure gradient, steric hindrance and wall friction. Correspondingly, the previous pore models were extended to different combinations for different factors. The relatively suitable model combination was determined through the comparison between the results of calculation and experiment. Then, all transport parameters of FOS were obtained based on the extended pore model. It also showed that the main factors influencing the transport property in nanofiltration membranes might be different considering different sugars. (c) 2004 Elsevier B.V All rights reserved.