In this paper, we proposed an online algorithm, called FQT-Stream (Frequent Query Trees of Streams), to mine the set of all frequent tree patterns over a continuous XML data stream. A new numbering method is proposed to represent the tree structure of a XML query tree. An effective sub-tree numeration approach is developed to extract the essential information from the XML data stream. The extracted information is stored in an effective summary data structure. Frequent query trees are mined from the current summary data structure by a depth-first-search manner. Categories and Subject Descriptors H.2.8 [Database Management]: Database Applications ? data mining. General Terms: Algorithms.