Chiral Mn(salen) complex 3 bearing a 1-ethyl-1-methylpropy1 group at C3, C3', C5, and C5' was found to induce higher asymmetry in the epoxidation of conjugated Z-olefins, especially less nucleophilic ones, in the presence of 4-phenylpyridine Noxide than complex 1 bearing a t-butyl group at the same carbons. The higher enantioselectivity was considered to be attributable to Me-in-plane conformation of the 1-ethyl-1-methylpropy1 group at C3 and C3'.