In existing educational systems, it is impossible to synchronize multimedia contents and it is required additional client applications. SMIL(Synchronized Multimedia Integration Language) is based on XML which allows to arrange multimedia objects temporally and spatially, and it can represent multimedia effectively on Web browsers. It is possible to serve the high-quality multimedia on Web browsers by applying SMIL to the previous Web-based educational contents without any client applications. In this paper, we propose the method of developing multimedia educational contents applied by SMIL, and design an authoring tool with a function of transforming SMIL documents to XHTML documents. The tool allows common users to use easily without any plug-ins or applications.