A new type of index structure called join predicate index (JPI) is described. The JPI can be used to improve the performance of joins in object relational data base systems. After the introduction of JPI, a join algorithm based on JPI is presented and the performance analysis formula is given. It is also proposed which should be the outer relation so as to decrease the expense of the join op-eration. Lastly, the basic JPI is extended to the join of n relations.