Abstract: UML is proposed as a unified language to both simulation object modeling and PADS software system modeling This work focuses on extending standard UML semantics to support simulation domain specific concepts including introducing virtual time in UML, and using UML statechart diagram and interaction diagram with virtual time labels and constraints to describe simulation entity functional property In addition, virtual time synchronization stereotypes are created to describe PADS time synchronization protocols, which separates the functional properties and synchronization properties, thus alleviating the simulation modeling complexity and improving both simulation model reuse and synchronization reuse A wireless network parallel simulation example is given to illustrate the extensions