The mechanism of the cycloaddition reaction of singlet silylene with formaldehyde has been studied by RHF/6-31G^* gradient method. The electron correlation energies of all the structures are calculated by using second-order Moller-Plesset perturbation theory (MP2). We have also calculated the changes of thermodynamic functions and the quality of kinetic properties for this reaction at different temperatures by using statistical thermodynamics method and transition-state theory. The results show that this reaction proceeds in two step: 1) silylene and formaldehyde form an intermediate complex, which is a kind of exothermal reaction with no barrier; 2) the intermediate complex isomerizes to sive the product. The barrier for the second step is 51.4kJ·mol^-^1 at MP2/6-31G^*//6-31G^* level (with zero-point energies correction). In view of dynamics and thermodynamics, it is between 300～400K that the reaction will have not only larger spontaneous tendency and equilibrium constant but also quicker reaction rate.

Key words: CYCLOADDITION REACTION, FORMALDEHYDE, SILYLENE, SINGLET, AB INITIO CALCULATION, THERMODYNAMIC PROPERTIES, KINETIC ANALYSIS, THERMODYNAMIC FUNCTION, REACTION MECHANISM, REACTION PATH