Abstract: Xylopyranose and O-acetyl-xylopyranose, the two monomers of xylan, were employed as the model compounds to study the mechanism for the formation of hydroxyacetaldehyde (HAA) from xylan by pyrolysis. Six possible pathways from xylopyranose and three from O-acetyl-xylopyranose were proposed by employing the density functional theory (DFT) at B3LYP/6-31+G(d,p) level; the energetically favored pathways for HAA formation were revealed. Xylopyranose may undergo ring-opening, dehydration, rearrangement and retro-aldol reactions sequentially, to form HAA that contains C4/C5; the rate-determining step is the dehydration reaction, with an energy barrier of 253.3 kJ/mol. From O-acetyl-xylopyranose, the side chain is cleaved in the first place, forming acetic acid (AA) and a cyclic intermediate; the ring-opening and H-shift reactions happen afterwards from the cyclic intermediate to generate HAA containing C4/C5; the rate-determining step is the H-shif reaction, with an energy barrier of 317.6 kJ/mol.