Autonomous, responsive and proactive nature of agents makes development of agent-based software systems more complex than other software systems. A Domain Specific Modeling Language (DSML) may provide the required abstraction and hence support a more fruitful methodology for the development of MASs especially working on the new challenging environments such as the Semantic Web. In this paper, we introduce a domain specific metamodel for MASs working on the Semantic Web. This new metamodel paves the way for definition of an abstract syntax and a concrete syntax for a future DSML of agent systems. Achieved DSML syntax is supported with a graphical modeling toolkit.