Detailed mechanisms of C4H4 + C4H4 reaction were investigated by accurate ab initio density functional theory B3LYP/6-311 + G(d,p) calculations, as well as CBS-QB3 calculations. It was found that styrene, phenylacetylene and 5-membered ring structure can be formed in C4H4 + C4H4 reaction through five parallel and competing pathways. Three pathways are featured with H-transfer and dehydrogenation reactions, and the other two pathways are characterized with the forming and breaking of C-C bond. The H-transfer reactions play a vital role in stabilizing intermediates. It was found that the H-transfer reaction is significantly dependent on the relative position of two C atoms, the degree of saturation of the second C atom, and the electronic environment of molecule. Generally, an ortho-position unsaturated C atom with relative small positive atomic charge is preferable in H-transfer reaction. The yield of products and reaction rate coefficients were evaluated by using Rice-Ramsperger-Kassel-Marcus theory with solving master equation at different combustion temperatures and pressures (T = 800-2500 K, P = 0.1-10 atm). The kinetic results indicate that styrene is the dominant product and the formation of other two products can be ignored when temperature is lower than 1600 K. The formations of phenylacetylene and 5-membered ring structure are only facilitated at higher temperature, but dependent on the combustion pressure. Specifically, phenylacetylene is favored by higher pressure and higher temperature, while 5-membered ring structure is favored by lower pressure and higher temperature. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.