Abstract: Four N'-(substituted-pyrimidin-2-yl)-N-2-chloro-phenoxyacetyl thioureas 2 were synthesized through the reactions of 2-chloro-phenoxyacetyl isothiocyanate with substituted-2-amino-pyrimidines.Four 2H-1,2,4-thiadiazolo2,3--a-pyrimidine dervatives were synthesized by the oxidizing cyclizations of 2. All of them are new compounds. Their structures were confirmed by IR, 1HNMR and elementary analysis.The preliminary herbicidal test showed that some of them have excellent inhibitory activities against root and stalk of dicotyledon plant, but selective to monocotyledon plant.