The reaction mechanisms of the butadiynyl radical (C4H) with ethylene (C2H4) to form H and C6H4 via hydrogen elimination are investigated using the density functional theory and high-level ab initio methods. The calculated geometrical parameters and dipole moment of the 2Π state of C4H are in excellent agreement with previously reported values using CCSD(T) with large basis sets. The calculated reaction enthalpy is also in excellent agreement with that of previously reported value. These results indicate that theoretical level in this work is optimal and more expensive calculations may not be necessary for the systems studied in this work. Eight isomers of C6H4 are considered in this work, and we present the highly complex reaction pathways by grouping them into three categories; (i) pathways including only chain intermediates without any ring components, (ii) pathways including ring formations except the six-membered ring, and (iii) pathways including the six-membered ring. On the basis of the calculated results, the most favorable reaction pathway is simple and found in the first category; H elimination from the initial chain adduct of C4H with C2H4 yields H and one of C6H4 isomers (CH2CHCCCCH). This reaction is similar to that of the reaction of ethynyl (C2H) radical with C2H4. This result clarifies the assumption in the recent experimental kinetics study. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011.