Abstract: This paper describes a practical process for the preparation of an active metabolite of roflumilast,with an overall yield of 19%.The polymorphic substances of roflumilast N-oxide were characterized by means of X-ray powder diffraction,IR,DSC,TGA and elemental analysis.Roflumilast N-oxide was found to exist in two crystal forms,form I and monohydrate form II Monohydrate crystal form II was further indentified by single-crystal X-ray structure analysis.