Eclipse is a Java integrated development environment (IDE) and tool integration platform that offers numerous extension points for customization through a plug-in architecture. This paper describes the design of an Eclipse plug-in called E-R Modeler. As a database design toolkit, the E-R Modeler provides an E-R (EntityRelationship) diagram development environment that supports XML and DDL (Database Definition Language) generation tools. It also provides database connection and schema creation capabilities. As an Eclipse plug-In, it offers multiple extension points for other applications to build upon. Categories and Subject Descriptors D.2.6 [Software Engineering]: Programming Environments – Integrated Environments. Keywords Eclipse, Plug-In, E-R diagram, Data Modeling, Design Patterns.