Abstract: A service provider based on open archive initiative/protocol for metadata harvesting is presented. Current status in searching among heterogeneous data in digital libraries is first discussed, and the general situation about OAI-PMH and the interoperation framework based on OAI-PMH are analyzed. A framework for service provider system based on OAI-PMH is designed and a push/pull model for updating data immediately in interoperation framework is proposed. Three main modules of the framework for service providers, viz. a batch of retrieval base URL of the data providers, metadata harvesting, and union search are designed and accomplished. Both metadata harvesting from distributed and heterogeneous data providers and union search based on unified metadata repository are implemented, so as to support both the transparency about information resource location for users and the interoperability among various digital libraries in the system. This provides a way of union search implementation among digital libraries.