Abstract: Methyl vinyl ketone oxide, an unsaturated four-carbon Criegee intermediate produced from the ozonolysis of isoprene has been recognized to play a key role in determining the tropospheric OH concentration. It exists in four configurations ( anti - anti , anti - syn , syn - anti , and syn - syn ) due to two different substituents of saturated methyl and unsaturated vinyl groups. In this study, we have carried out the electronic structure calculation at the multi-configurational CASSCF and multi-state MS-CASPT2 levels, as well as the trajectory surface-hopping nonadiabatic dynamics simulation at the CASSCF level to reveal the different fates of syn / anti configurations in photochemical process. Our results show that the dominant channel for the S _1 -state decay is a ring closure, isomerization to dioxirane, during which, the syn (C - O) configuration with an intramolecular hydrogen bond shows slower nonadiabatic photoisomerization. More importantly, it has been found for the first time in photochemistry of Criegee intermediate that the cooperation of two heavy groups (methyl and vinyl) leads to an evident pyramidalization of C3 atom in methyl-vinyl Criegee intermediate, which then results in two structurally-independent minimal-energy crossing points (CIs) towards the syn (C - O) and anti (C - O) sides, respectively. The preference of surface hopping for a certain CI is responsible for the different dynamics of each configuration.