The ring expansion reaction mechanisms between cyclopropenylidene and cyclic CnH2nO (n = 2, 3) compounds have been systematically investigated employing the second-order Møller-Plesset perturbation theory (MP2) method in order to better understand the reactivity of cyclopropenylidene with epoxy compounds (oxirane and epoxypropane). Geometry optimizations and vibration analyses have been performed for the stationary points on the potential energy surfaces of the system. The calculated results show that cyclopropenylidene can insert into oxirane at its CO bond and into epoxypropane at its CO or CC bond. From the kinetic viewpoint, it is easier for cyclopropenylidene to insert into the CO bond of oxirane than that of the CO bond of epoxypropane. For insertion into epoxypropane, it is easier for cyclopropenylidene to insert into its CO bond than CC bond. Through the first insertion step and the second ring-opened step, spiro and carbene intermediates can been formed between cyclopropenylidene and epoxy compounds, respectively. Through the following two H-transfer steps, carbene intermediate forms the products of allenes and alkynes, respectively. From the thermodynamics viewpoint, the allenes are the dominant product for the title reaction.