In the conversion of methanol to olefins (MTO), it is now well accepted that ethene and propene are formed via a direct mechanism in the initial stage; after a transition period, olefins are then mainly produced via a hydrocarbon pool (HCP) mechanism in the steady stage. However, there was still no clear consensus over what actually occurred in the transition period and how the initial ethene and propene products construct the first HCP intermediates. In this work, the formation of initial HCP intermediates in the transition period and their evolution tendency were investigated for MTO over H-ZSM-5 zeolite by density functional theory and microkinetic modeling. The results illustrate that ethene and propene formed in the initial stage can quickly construct methylcyclopentadiene and methylbenzene; acting as the initial HCP intermediates, these cyclic species can build the light olefins through both the pairing route and the side-chain route and then promote the MTO reactions into the steady stage governed by the HCP mechanism. Meanwhile, the formation of methylcyclopropene and methylnaphthalene needs to conquer a high free energy barrier, which should be less significant as the HCP intermediates in the transition period. The insight attained in this work helps to clarify the origin and evolution of initial cyclic HCP intermediates in the transition period and is thus beneficial for a better understanding of the MTO reaction mechanism. (C) 2018 Elsevier Inc. All rights reserved.