Effective integration of heterogeneous data sources has been studied as the most pressing challenge in various fields; such as, high energy physics, astronomy, and life sciences. In this talk, we present a data integration system by using Globus Toolkit with OGSA-DAI. For associating related data among many databases, we have introduced metadata based on their domain ontologies. Using the system one can make a database access flow for describing a set of queries as a workflow, and can query across the databases without aware of their locations and schemas.