The mechanism of the cycloaddition reaction of singlet dimethyl methylene carbene and acetone has been studied by using second-order Moller-Plesset perturbation and density functional theory. The geometrical parameters, harmonic vibrational frequencies and energy of stationary points on the potential energy surface are calculated by MP2/6-31G* and B3LYP/6-31G* methods. The results show that path b of the cycloaddition reaction (1) would be the major reactive channel of the cycloaddition reaction between singlet dimethyl methylene carbene and acetone, which pro- ceeds in two steps: i) The two reactants form an energy-rich intermediate (INT1b), which is an exothermic reaction of 23.3 kJ/mol with no energy barrier. ii) The intermediate INT1b isomerizes to a three-membered ring product (P1) via transition state TS1b with energy barrier of 22.2 kJ/mol. The reaction rate of this reaction and its competitive reactions do greatly differ, with excellent selectivity. In view of dynamics and thermodynamics, this reaction is suitable for occurring at 1 atm and temperature range of 300―800 K, in which the reaction will have not only the larger spontaneous tendency and equilibrium constant but also the faster reaction rate. The mechanism of the cycloaddition reaction of singlet dimethyl methylene carbene and acetone has been studied by using second-order Moller-Plesset perturbation and density functional theory. The geometrical parameters, harmonic vibrational frequencies and energy of stationary points on the potential energy surface are calculated by MP2 / 6-31G * and B3LYP / 6-31G * methods. The results show that path b of the cycloaddition reaction (1) would be the major reactive channel of the cycloaddition reaction between singlet dimethyl methylene carbene and acetone, which pro- ceeds ii) The intermediate INT1b isomerizes to a three-membered ring product (P1) in two steps: i) The two reactants form an energy- rich intermediate (INT1b), which is an exothermic reaction of 23.3 kJ / mol with no energy barrier. via transition state TS1b with energy barrier of 22.2 kJ / mol. The reaction rate of this reaction and its competitive reactions do slight differ, with excellent selectivity. In view of dynamics and ther modynamics, this reaction is suitable for occurring at 1 atm and temperature range of 300-800 K, in which the reaction will have not only the greater spontaneous tendency and equilibrium constant but also the faster reaction rate.