Ab initio electronic structure calculations at 6 31G ** level were performed to determine the mechanism of the isomerization reaction from 1,2 butadiene to 1,3 butadiene. The MP2 method was used to improve the barrier energies of the reaction. The calculation results show that the reaction is a stepwise process via a radical INT; the barrier energies determined for TS1 and TS2 at UMP2/6 31G ** //UHF/6 31G ** +ZPE level are 64 95×4.184 and 64.10×4.184 kJ/mol, respectively. Based on the unified statistical theory, the thermal rate constants for the present complex reaction mechanism were computed over the termperature range 1 100_1 600 K. The computed thermal rate constants for the reaction are well represented by the following Arrhenius expression: k(T)=2.4×10 13 exp (-62 8 kJ/RT) s -1 . The theoretical investigations are found to be in quantitative agreement with the experimental results.