In the presence of a bis (1, 3-diketonato) nickel (II) complex, various olefins are directly monooxygenated into the corresponding epoxides on treatment with atmospheric pressure of oxygen or air and aldehydes. Especially, it is noted that bis [1, 3-bis (p-methoxyphenyl)-1, 3-propanedionato] nickel (II) behaves as an excellent catalyst in the present epoxidation.