A cinnoline derivative having the formula (I): ##STR1## in which X is --OH, --O.sup.-M.sup.+, --OR.sup.1 or ##STR2## wherein M.sup.+ is an alkali metal cation, an alkaline earth ##STR3## in which R.sup.4, R.sup.5 and R.sup.6 are the same or different and each is hydrogen atom, a C.sub.1 -C.sub.6 alkyl group, a C.sub.3 -C.sub.4 alkenyl group, a C.sub.3 -C.sub.4 alkynyl group, a C.sub.3 -C.sub.8 cycloalkyl group, benzyl group or phenyl group; R.sup.1 is a C.sub.1 -C.sub.9 alkyl group, a C.sub.3 -C.sub.6 alkenyl group, a C.sub.3 -C.sub.4 alkynyl group, a C.sub.1 -C.sub.3 alkoxy (C.sub.1 -C.sub.4) alkyl group, a C.sub.1 -C.sub.3 haloalkyl group, a C.sub.3 -C.sub.8 cycloalkyl group, benzyl group or phenyl group; and R.sup.2 and R.sup.3 are the same or different and each is hydrogen atom, a C.sub.1 -C.sub.6 alkyl group, a C.sub.3 -C.sub.4 alkenyl group, a C.sub.3 -C.sub.4 alkynyl group, a C.sub.3 -C.sub.8 cycloalkyl group, a benzyl group in which at most two of hydrogen atoms at the .alpha.-position thereof may be substituted by methyl group, a C.sub.2 -C.sub.3 hydroxyalkyl group or a phenyl group in which at most three of hydrogen atoms thereof may be substituted by the same or different C.sub.1 -C.sub.2 alkyl group or halogen atom;Y is fluorine atom, chlorine atom, bromine atom, a trihalomethyl group, a C.sub.1 -C.sub.6 alkoxy group or a C.sub.1 -C.sub.2 alkyl group;A is a C.sub.1 -C.sub.3 polyhaloalkyl group; andA' is hydrogen atom, fluorine atom, chlorine atom or bromine atom,a process for preparing the same, and a herbicidal composition containing the same as an active ingredient,a method for controlling undesired weeds using the same, and use of the same as a herbicide.