In a heterogeneous database system which consists of object databases, a global schema created by integrating schemas of the component databases can provide a uniform interface and high level location transparency for the users to retrieve data. The main problem for constructing a global schema is to resolve conflicts among component schemas. In this paper, we define corresponding assertions for the database administrators to specify the semantic correspondences among component object schemas. Based on these assertions, integration rules are designed, which use a set of primitive integration operators to restructure the component schemas for resolving the conflicts and do the integration. The principle of our integration strategy is to keep the data of component databases retrievable from the global schema without losing information. Moreover, more informative query answers may be derived from the heterogeneous databases due to schema integration. Finally, the mapping strategy between global schema and component schemas is provided.