The cracking mechanism of 1-hexene over H-ZSM-5 zeolite has been studied by using the ONIOM(B3LYP/6-31G(d,p):UFF) method with an 88T cluster model and the B3LYP/6-31G(d,p) method with a 5T cluster model. The computation results show that the cracking process follows a two-step mechanism involving a carbenium ion intermediate. When the 88T cluster model is used, a stable carbenium ion can be found. When the 5T cluster model is used, no stable carbenium ion is obtained, however, the intrinsic reaction coordinate prompts that there exists a carbenium ion intermediate during the reaction process. The theoretical researches indicate that the H-ZSM-5 zeolite environment plays a significant role in stabilizing the hexyl carbenium ion. The adsorbed carbenium ion on the surface of zeolite is an active high energetic species, and the cleavage of the C-C bond on beta position occurs easily. The extended zeolite framework has a large effect on the energetics of the adsorbed complexes. The apparent energy barrier of the cracking reaction with 88T cluster model is 104.73 kJ/mol and that with 5T cluster model is 149.50 kJ/mol. (c) 2013 Elsevier B.V. All rights reserved.