The mechanism of altering the rate-limiting step in enediyne cycloaromatization (Bergman reaction) by benzannelation has been studied using ab initio molecular orbital (MO) methods. The calculated results indicate that the benzannelation effect cannot be interpreted by the energy separation between the lowest singlet and triplet states in p-benzyne-type intermediates 2 and 4, as revealed for p-benzyne 6 and 1,4-diylnaphthalene 7. The energy barriers for the retro-cyclizations of the intermediates to produce cyclodec-3-ene-1,5-diyne (1) and 3,4-benzo-cyclodec-3-ene-1,5-diyne (3) are estimated to be 15.3 and 5.9 kcal/mol, respectively. Hydrogen abstractions from methane by p-benzyne-type intermediates in the Bergman reaction of 1 and 3 are calculated to have energy barriers of 12.7 and 11.8 kcal/mol. From these theoretical results on the energy barriers of retro-cyclizations and hydrogen abstraction, we concluded that the rate-limiting step in Bergman reaction of 3 is hydrogen abstraction rather than cyclization, while that of 1 is cyclization.