Building graphical models and running simulations directly on the models provides an attractive way of performing simulation. Most existing network-based simulation systems use graphical models to represent real world systems; however, users have to create separate simulation input files (usually text) to run the simulation. Inconsistency and mistakes between the two representations of a model are common and difficult to detect. By using object-oriented programming, modeling elements of a simulation system can be designed as objects that combine both interactive graphics and discrete-event simulation fi,mctions. This integration provides n@ only the friendly user interface to build simulation models but also the desired direct simulation on graphical networks, COOPS is a new discrete-event simulation system that utilizes this integration concept. It is a PC-based general purpose discrete-event simulation system capable of modeling systems such as service and manufacturing systems, or construction operations. COOPS runs on computers with 386, 486 or better CPU on top of Microsoft Windows 3.0 or above. This paper presents the modeling concept of COOPS and the design that makes the integration possible.