Enantioselectiveaerobicepoxidation of acyclic cis-olefins into the corresponding opticallyactive epoxides was carried out with good-to-high enantioselectivities by using a new class of opticallyactiveβ-ketoiminatomanganese(III) complexcatalyst, N,N′-ethylenebis(α-mesitoyl-β-ketoiminato)manganese(III) chloride. The key intermediate of the present epoxidation is also discussed.