Abstract: 4-Fluoro-3-hydroxy-1-substituted phenyl-6-pyridazinones were prepared by the condensation of fluoro maleic anhydride ( Ⅰ ) with substituted phenyl hydrazines [Substituents: R(Ⅱ)=m-CF₃; R(Ⅲ) = p-CH₃; R(Ⅳ)=o-CH₃; R_(Ⅴ) = p-Br]. It was identified that compounds ( Ⅱ ), (Ⅲ), (Ⅳ) and (Ⅴ) exist in the form lactim and the fluorine atoms in these compounds are at C₄ position in pyridazine ring. The reaction of ( Ⅰ ) with m-trifluoro methyl phenyl hydrazine in dilute hydrochloric acid gave N-(m-trifluoro methyl anilino)-fluoro maleimide (Ⅳ) which can be converted into ( Ⅱ ) in concentrated hydrochloric acid. The reaction of (Ⅲ) and (Ⅴ) with phosphorus oxy chloride gave corresponding₃,4-dichloro substituted compounds (Ⅶ) and (Ⅷ) respectively. The reaction of (Ⅲ) with methyl sulfate gave 3 ,4-dimethoxy-l-(p-tolyl)-6-pyridazinone(Ⅸ ). Compounds ( Ⅱ ), ( Ⅲ ) and (Ⅷ) have a good effect on preventing the rice-blast, anti-intestinal spasm and tuberculostatic activity, respectively.