Unified Modeling Language (UML) is the standard notation technique used for modeling object-oriented (OO) systems. Previous efforts exist to extend UML for representing multi-agent systems (MAS). Due to the UML widespread, the resulting extensions are therefore easy to use and to adopt by software engineers. Unfortunately, most of these attempts are based on applying stereotypes for the object-oriented entities. However, heavyweight extension is a more adequate choice to handle different capabilities of the MAS over the OO systems. In this work a heavyweight extension to UML metamodel is proposed for modeling MAS according to our unified MAS conceptual model. It is not limited to a specific application domain or specific agent architecture. A proposed MAS-UML tool is built based on the extended metamodel.