We study the isomerization and dissociation of syn-CH3CHOO with high internal energies by combining electronic structure calculations, ab initio molecular dynamics, and RRKM microcanonical variational transition-state theories. The results show a striking effect of the internal energy on the reaction fate of syn-CH3CHOO. With lower internal energies, syn-CH3CHOO prefers isomerizing to vinyl hydroperoxide, which then produces hydroxyl radical. As the available internal energy increases, the O-elimination channel becomes more competitive and eventually the dominant channel. A third channel leading to methyl dioxirane is found to be less competitive than the former two. However, this channel is highly exothermic and produces greenhouse gases CH4 and CO2; thus, further investigation is needed to quantify its potential impact on the atmosphere. The present study provides new insight on clarifying the unimolecular reaction fate of hot syn-CH3CHOO and theoretical solutions for revealing the impact of internal energy on the complex gas phase reactions.