The mechanism of the abstraction of oxygen from epoxide by carbenes has been investigated using the B3LYP/6-31G* and MP2/6-311 + + G(d,p)//B3LYP/6-31G* levels of theory. According to our model calculations, the reactivity of carbene decreases in the order: four-membered ring carbene > six-membered ring carbene > three-membered ring carbene > five-membered ring carbene much greater than vinylidene. Moreover, the present work shows that the singlet-triplet splitting of a carbene can be used as a guide for the prediction of its reactivity.