An imidazolidine-2-one derivative represented by the formula ##STR1## wherein R.sup.1 represents hydrogen atom, lower alkyl group, lower alkoxy lower alkyl group, benzyl group, lower alkoxycarbonyl lower alkyl group or lower alkylcarbamoyloxy group and R.sup.2 represents a group --Ar or group --CONH--Ar wherein Ar represents a phenyl group, phenoxyphenyl group or aralkyloxyphenyl group each of which may be substituted with a halogen atom, nitro group, lower alkyl group, lower alkoxy group or haloalkyl group, a process for preparing the same and a herbicidal composition containing the same.