Abstract: As semi-structured data, XML brings some problems in the normalization of Native XML Database data schema. To solve the problems, the method of combination of xml structure information and xml data information was proposed, and by using this method the concept xml functional dependency was defined. The concept of xml schema path, xml tree node path and the formalization of a validated xml document were studied. On the basis of these concepts, the definition of tree tuple and equation of tree tuple were constructed. As a result, the definition of xml functional dependency was proposed on the basis of above-mentioned concepts. The inference rules about xml functional dependency includes reflectivity, augmentation and transitivity were proved to deduce the implication of xml functional dependency in a given sets. The rules were proved right. Our work provides some theory basis of the normalization of native XML database data schema, and also enhances the semantic of xml data.