Abstract: With the purpose of seeking more effective, less hazardous bactericides against rice leaf blight, there were synthesized sixteen 2-amino-5-alkyl-l,3, 4-thiadia-zoles (Ⅰ) (among them seven new compounds) and eleven O,O-diphenyl phosp-horyl amides of various 2-amino-5-alkyl-1,3,4-thiadiazoles (Ⅱ), as well as four O,O-dialkyl-S-(N-l,3,4-thiadiazole) aminomethyldithio-phosphorates (Ⅲ). All these compounds were tested against xanthomonas campestris pv.oryzae or Agrobacterium tumcufacenec in vitro. The biological evaluation results showed that compounds of types Ⅰ and Ⅱ were active and the activity was correlative with the length of the n-alkyl at 5 position.