The mechanism of the isomerization of the enol acetate of 2-aceto-1,3-cyclohexanedione has been discussed in detail. The possible 1,3-H and 1,5-H shifts in isomerization were investigated systematically. It seems that this mechanism includes two successive 1,5-sigmatropic shifts, i.e. 1,5-acetyl and 1,5-H shifts. Density functional theory calculations have been performed to evaluate the reasonability of the proposed mechanisms. The effect of the solvent upon the rate-determining steps has been also considered. In addition, the relative stabilities of the reactant, the product as well as the intermediates in the proposed mechanism have been examined and discussed.keywords density functional theory, isomerization mechanism, hydrogen shift, 2-aceto-1,3-cyclohexanedione, solvent effect