The mechanism for the thermal isomerization reactions of 2-methyl-4,5-dihydrofuran was investigated using quantum chemical calculations at B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), and [PBE/6-31G(d,p), PBE/6-31++G(d,p)] levels of theory. It was found that 2-methyl-4,5-dihydrofuran isomerizes to acetylcyclopropane, and by a parallel reaction a slower isomerization to give 3-pentene-2-one. The acetylcyclopropane formation occurs through unimolecular electrocyclic mechanism. The 3-penten-2-one formation also takes place through electrocyclic mechanism, involving [1,2] hydrogen migration. The isomerization reaction of acetylcyclopropane to 3-penten-2-one occurs by step-wise mechanism, with the formation of an intermediate product 2-hydroxy-2,4-pentadiene, which subsequently isomerizes to the keto form, 3-penten-2-one. The step-wise acetylcyclopropane isomerization to 3-penten-2-one has lower energy of activation than the direct conversion of 2-methyl-4,5-dihydrofuran to 3-penten-2-one. Reasonable agreement was found between experimental and calculated energies of activation using B3PW91/6-311G(d,p) and MPW1PW91/6-311G(d,p) methods. Results suggest that both isomerization reactions pathways are possible under the experimental conditions reported. However, the lower energy of activation of the rate determining step of the step-wise mechanism favors this process over the single step mechanism.