Set type is an important data type in object-oriented database system and object-relational database system. An index structure of set type Set_struc is presented in this paper. In Set_struc all sets are organized as a tree, and the sets with common prefix are merged. So the size of the index will be decreased for the data set with a large number of repeated data and frequent patterns. Based on Set_struc, an algorithm of join operation with Set_struc is presented. Its performance is better than other methods such as PSJ (partition based join).