With the development of WWW technology, web-based ITSs are becoming mainstream area of research and development. The major benefit of web-based ITS is that, the ITS installed and supported in one place can be used by thousands of learners all over the world. Although many web-based educational systems appeared recently, most of them emerged from their predecessor legacy standalone systems. Therefore, they not only restrict themselves in functionality, but also fail to take advantage of distributed nature of Internet. This paper describes an open architecture based adaptable web-based intelligent tutoring system with pluggable domain modules. The system is based client/server architecture and has distinct and separable domain modules and a generic module. Such architecture not only provides salability in the Internet environment but also allows the same architecture to be used for multiple subject domains.