Abstract: An index structure, tendency B trees, to alleviate the high cost of string partial search in large data sets is presented. A tendency B tree is a two layered data structure, including a logical layer and a physical layer. In the logical layer, a tendency tree provides a hierarchical structure to group similar tendency features together to facilitate fast partial search for a given query. The physical layer is a B-tree like structure. In addition, the balanced topology of B trees provides consistent I/O complexity. The tendency B tree is dynamically compressed during the construction process to reduce storage and enhance search efficiency. Experiments on both dictionary search and DNA sequence search using tendency B trees show that consistent, fast search times can be achieved in large data sets, requiring lower space usage and linear construction time.