Abstract: 4-Fluoro-1,2-dihydro-3,6-pyridazindione( Ⅱ ) and 1-phenyl-3-hydro-xy-4-fluOro-6-pyridazinone (Ⅶ) were prepared by reaction of fluoro maleic anhydride with hydrazine hydrate and phenyl hydrazine, respectively. (Ⅱ) reacted with phosphorus oxychloride to afford 3,6-dichloro-4-fluoro-pyridazine( Ⅲ ). ( Ⅲ ) reacted with sodium methoxide, in equal molar ratio, to give 3-methoxy-4-fluoro-6-chloro-pyridazine (Ⅳ), while in 1 to 3 ratios, to give 3,4-dimethoxy-6-chloro-pyridazine (Ⅴ). (Ⅲ) reacted with sodium o-metnyl-phenoxide, to give 3-o-methyl-phenoxy-4-fluoro-6-chloro-pyridazine(Ⅵ). In(Ⅲ), nucleophilic displacement is taken place preferably at 3th position. (Ⅱ), (Ⅲ), (Ⅳ), (Ⅵ) and (Ⅶ) are new compounds. (Ⅱ) has a plant growth regulator property and (Ⅶ) has an anticonvulsemt effect.