This paper presents an approach to a simulation-based design methodology and is motivated by the need to provide models representation schemes and simulation software tools for combined discrete event-continuous systems. Our approach is based on an extension of the DEVS (Discrete EVent System Specification) introduced by Zeigler, to include a combined discrete event-continuous systems. We show how the interactions between the discrete-event part and the continuous part and their consequence on the system behavior can be expressed. By employing the Object-Oriented paradigm, the extended model is implemented as an abstract class, in which the methods describe the model's behavior and data represent its variables.