The most stable stereo conformer in non substituted benzenesulfonyl urea, 1 was the II-keto form, which the molecule was intramolecular associated(H-bond) coformer between imide group and N atom on the Pyrimidine ring. The hydrolytic degradation of 2 derivatives were proceeds by nucleophilic addition reaction(p<0) with orbital controlled intermolecular interaction between LUMO with electron donating$(sigma<0)$ groups of 2 and HOMO of water molecule. N-(4,6-disub. pyrimid ine-2-yl)aminocarbonyl-2-(1,1-dimethoxy-2-fluoro)ethylbenze nesulfonamides,3 and N-(4,6-disub. triazine-2-yl)aminocarbonyl-2-(1,1-d imethoxy-2-fluoro)ethylbenzenesulfonamides,4 we re synthesized and their herbicidal activities in vivo against bulrush (Scirpus juncoides.) were measured by the pot test under the paddy conditions And the structure activity relationships(SAR) were analyzed by the multiple regression technique. The results of the SAR suggested that the 3 and 4 derivatives indicated dependent on the hydrophobicity of the 4,6-disubstituents and the heterocyclo group, where the optimal value $((log;P)_{opt.}=0.89)$ of hydrophobicity was 0.89. The pyrimidine substituents, 3 showed higher herbicidal activity than the triazine substituents, 4. Among them, 4,6-dimethoxypyrimidine substituent, 3a showed the best herbicidal activity.