The mechanism of fast pyrolysis of cellulose has been studied by using an analytical pyrolyzer coupled with a gas chromatography-mass spectrometry set-up (Py-GC/MS). The results showed that the main products comprised pyrans such as levoglucosan and levoglucosenone, furans such as furfural and 5-hydroxymethyl furfural, and linear small molecular chemicals such as acetaldehyde and 1-hydroxy-2-propanone. The compositions of products from fast pyrolysis of cellubiose and glucose were similar to that from cellulose, but with higher furan contents and lower pyran contents. Based on the experimental results, density functional theory (OFT) studies were carried out to deduce the pyrolysis mechanism of cellulose. The results showed the formation of 5-hydroxymethyl furfural from D-glucopyranose unit to be easier than the formation of levoglucosan, in agreement with the experimental results. The deduced mechanism of reaction pathways in cellulose pyrolysis provides insight into the pyrolysis behavior of cellulose and allows modification of previously proposed related mechanisms. (C) 2011 Elsevier Ltd. All rights reserved.