In this study, the initial reaction of lignin pyrolysis was explored in depth with light gas emission combined with liquid bio-oil composition and physical-chemical structure of solid bio-char. The result reveals that lignin pyrolysis undergoes a complex initial reaction between 160 and 330 degrees C with significant mass loss (about 20%). Lignin molecular network begins to crack down with lignin monomer linkage breaking and light gas molecular (CO2 and CO) formation as temperature reaching 160 degrees C. Also, polymerization reaction involving forming larger molecular weight lignin pieces took place between 160 and 200 degrees C. After that (200-330 degrees C), lignin cracking reaction is the main reaction with lignin molecular accelerating to form heavy (dimers and trimers) and light units (monomers) in bio-oil and causing more mass loss. At the same temperature range, lignin soften reaction happens and results in lignin surface adhesion, microscopic surface structure changing and functional group evolution. As temperature over 330 degrees C, lignin initial pyrolysis stage finished and entered intense pyrolysis stage, with more oxygen contained functional group cracking and light lignin pieces formed. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.