One approach to achieving high performance in the DBMS in the critical application is to store the database in main memory rather than on disk. One can then design new data structures and algorithms oriented towards increasing the efficiency of the main memory database -MMDB. In this paper we present some results on index structures from an ongoing study of MMDB. We propose a new index structure, the T-tail Tree. We give the main algorithm of the T-tail Tree and the performance of these algorithms. Our results indicate that T-tail Tree provides good overall performance in main memory.