Density functional theory was employed to investigate gasification mechanism of beta-5 linkage lignin in high temperature steam at B3LYP/6-31G++(d,p) level. Four possible decomposition reaction pathways and four successive reactions pathways were proposed. The equilibrium geometries of the reactants, transition states, intermediates and products were optimized. The activation energies, Gibbs free energy changes and enthalpy changes of each reaction step in gasification pathways were analyzed. From the calculation results of Delta H-theta, it is concluded that the four decomposition reaction are endothermic below 1300 K, and exothermic exceed 1500 K. Furthermore, the Delta H-theta of each reaction decrease with temperature increasing. From the Delta G(theta) calculation results, Path 2 and Path 3 occur spontaneously without temperature limited. Path 1 and Path 4, however, are spontaneous reaction above 700 K. It is not ensure that Path 5, Path 6, Path 7 and Path 8 can occur spontaneously. Comparison of activation energy of each reaction pathway, Path 2 is optimal decomposition reaction without a competitive reaction. The main products in decomposition reaction are 5-hydroxyphenyl-acetaldehyde (P5) and 12-hydoxymethyl-phenol (P4). Path7 is the optimal subsequent reaction with a competitive reaction, i.e., Path 5. The main products in subsequent reaction are CO, phenol and formaldehyde. (C) 2017 Elsevier Ltd. All rights reserved.