Abstract: An XML-based and ontology-supported knowledge description language (KDL) was first presented, which has three-tier structure(core KDL, extended KDL and complex KDL), and takes advantages of strongpoint of ontology, XML, description logics, frame-based systems. The XML-Based syntax of KDL was introduced, and the methods of translating KDL into first order logic (FOL) was given. The implementation of KDL on the Web was described, and the reasoning ability of KDL proved by experiment was illustrated. It is demonstrated by practice that KDL encompasses XML and resource description framework(RDF) model properties, and KDL has powerfully reasoning ability.