Abstract: Separation performance and purification of xylo-oligosaccharides (XOs) solution derived from wheat straw by a nanofiltration(NF) membrane with a molecular weight cut-offs (MWCO) of 250 were investigated. The profiles of rejections of the arabinose, xylose and XOs with degree of polymerization (DP) from two to four at different membrane fluxes were obtained based on the detection of their concentrations in filtrate and retention. According to irreversible thermodynamics model, reflection coefficient, solute permeability, solution hydraulic permeability of arabinose, xylose and XOs with degree of polymerization (DP) from two to four were estimated. Furthermore, the membrane structure parameters were estimated by Steric-hindrance pore model. When feed concentration (total saccharide/syrup), membrane system temperature and trans-membrane pressure were 8.4%, (30.1±2.1)℃ and (1.58±0.02)×106 Pa, respectively, the flux was (29.4±1.8) L/(m2·h) without declination during diafltration, and the ultimate XOs content reached 92.54% (XOs/total saccharide) from 83.33%. The yield of total saccharide and the loss of xylobinose were 88.39% and 12.45%, respectively, after the addition of three times volumes of water. This investigation provides a basis of parameters for purification by diafiltration, concentration and optimization of those processes. Diafiltration with NF can efficiently remove monosaccharides with less loss of XOs.