A novel .DELTA..sup.2 -1,2,4-triazolin-5-one derivative having a herbicidal activity, which is represented by the formula ##STR1## wherein, R.sup.1 is a C.sub.1 -C.sub.4 alkyl; R.sup.2 is a hydrogen atom, a C.sub.1 -C.sub.6 alkyl group, or a C.sub.2 -C.sub.4 alkenyl group; and X is a hydroxy group, a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.6 alkyloxy group, an alkyloxyalkyloxy group of which two alkyls may be same as or different from each other and each alkyl is of C.sub.1 -C.sub.4, a C.sub.2 -C.sub.4 alkenyloxy, or an alkyloxycarbonylalkyloxy group of which two alkyls may be same as or different from each other and each alkyl is of C.sub.1 -C.sub.4.