In this work the effect of particle size and of the severity factors were investigated to find the optimum condition of steam explosion pretreatment on xylan recovery of beech wood. The beech wood particle with sizes of 0.16, 1, or 2 mm were steamed at 150°C-210°C for 2.5-15 minutes before the explosive decompression. The results showed that the maximum xylan recovery was about 10% w/w wood with low concentration of the inhibitors, which were obtained when the particle size is 1 mm and R 0 = 3.65 (190°C, 10 min). The smaller particle size may result in overcooking of biomass, lead to easily and high degradation of hemicellulose sugars, whereas the larger particle size may result in incomplete autohydrolysis in biomass and lower extractability of hemicellulose sugars. The obtained optimum condition for xylan recovery will improve the subsequent utilization (such as in food industry and other chemical products), prior to subsequent transformation of steam explosion pretreated wood (bioethanol and pellet). Keywords: Steam explosion pretreatment; beech wood; severity factor; biomass particle size; xylan recovery. Graphical abstract Statement of Novelty : The steam explosion pretreatment of beech sawmill has been studied. The effect of hardwood chips particle size on the sugars recovery (xylose and xylans) in the liquid stream has been examined for the first time. The final goal is the improved valorization of the xylans from hardwood sawmill waste.