The potential energy surfaces of Mn+ reaction with ethylene oxide in both the septet and quintet states are investigated at the B3LYP/DZVP level of theory. The reaction paths leading to the products of MnO+, MnO, MnCH2+, MnCH3, and MnH+ are described in detail. Two types of encounter complexes of Mn+ with ethylene oxide are formed because of attachments of the metal at different sites of ethylene oxide, i.e, the O atom and the CC bond. Mn+ would insert into a C-O bond or the C-C bond of ethylene oxide to form two different intermediates prior to forming various products. MnO+/MnO and MnH+ are formed in the C-O activation mechanism, while both C-O and C-C activations account for the MnCH2+/MnCH3 formation. Products MnO+, MnCH2+, and MnH+ could be formed adiabatically on the quintet surface, while formation of MnO and MnCH3 is endothermic on the PESs with both spins. In agreement with the experimental observations, the excited state a(5)D is calculated to be more reactive than the ground state a(7)S. This theoretical work sheds new light on the experimental observations and provides fundamental understanding of the reaction mechanism of ethylene oxide with transition Metal cations.