This paper describes our participation in the GeoCLEF monolingual English task of the Cross Language Evaluation Forum 2006. The main objective of this study is to evaluate the retrieve performance of our geographic information retrieval system. The system consists of four modules: the geographic knowledge base that provides information about important geographic entities around the world and relationships between them; the indexing module that creates and maintains textual and geographic indices for document collections; the document retrieval module that uses the Boolean model to retrieve documents that meet both textual and geographic criteria; and the ranking module that ranks retrieved results based on ranking functions learned using Genetic Programming. Experiments results show that the geographic knowledge base, the indexing module and the retrieval module are useful for geographic information retrieval tasks, but the proposed ranking function learning method doesn't work we...