The diffusive-thermal effect plays an important role in the intrinsic instability of premixed flames. A two-dimensional direct numerical simulation of the propagation of premixed hydrogen-air flames was performed using a detailed chemical kinetics model. The cellular behavior of a lean hydrogen-air flame was analyzed on the basis of its chemical structure. The primary consequence of the diffusive-thermal effect was found to be a change in the buildup process of reactive intermediates by the chain reaction mechanism at the preheat zone. The resultant chemical structure at the main reaction zone can be explained by the local composition change of the gas flowing into the reaction zone from the preheat zone. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.