“Associate-Select”compound operation．which is similar in functionality to join operation of multi-nested relations in nested relational databases，is a key issue of query optimization in object-oriented database systems．This paper proves that the optimum solution for treelike“Associate-Select”queries with some restrictions can be obtained with polynomial complexity，then presents some algorithms to deal with the situation when birelationship exists in complex objects(for example by defining inverse attribute et a1．)．A simulation experiment is performed to demonstrate the steadiness and effective ness of the algorithms proposed in the paper．