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Interval index structure plays an important role in constraint database systems. A dynamic interval index structure DM-tree is presented in this paper.The advantage of the DM-tree compared with other interval index structures is that the dynamic operations of insertion and deletion can be operated on the new structure. The storage complexity of the tree is O(n), and the query I/O complexity is O(logn+t/B). To improve the performance of the inserting and deleting operations,some methods such as neighbored-constraint and update-late are applied. The I/O complexity of inserting and deleting operations is the same as that in B-tree, i.e.,O(log n).