Novel 5-substituted -3(2H)-pyridazinone derivatives and preparation thereof are provided.The derivatives have the formula: ##STR1## wherein, R denotes an alkyl, R.sup.1 and R.sup.2 denote each independently hydrogen or a lower alkyl, X denotes hydrogen, a lower alkyl, a halogen, a lower alkoxy, a lower haloalkyl, a lower haloalkoxy, a cycloalkyl, trimethylsilyl (-Si(CH.sub.3).sub.3), ##STR2## (wherein Y denotes hydrogen, a halogen, a lower alkyl, a lower alkoxy, a lower haloalkyl, a lower haloalkoxy or a cycloalkyl, and m is an integer of 1 to 5), n is an integer of 1 to 5, and Z denotes a halogen; said X being the same or different when n is 2 or more, and the alkyls and the alkyl moieties contained in the groups being straight or branched.