A novel and effective synthesis of pyridazino [4, 5-b][1, 4] oxazin-3, 8-diones via Smiles rearrangement is presented. Treatment of N-substituted 2-chloro (or hydroxy) acetamide, 2- tetrahydropyranyl-4-chloro-5-hydroxy (or chloro) pyridazin-3-one and cesium carbonate in refluxing acetonitrile was afforded the corresponding pyridazino [4, 5-b][1, 4] oxazin-3, 8- diones in excellent yield.