This paper discusses the successive steps used for modeling a real-time process control system. The term "process control" means the control of a process by a -computer-. The "real time process" is distinguished from other types of processes by two basic features the fast response to sudden external events is an essential criterion, and - the order in which the external events are occurir,g is the main critical factor [3] These two private characteristics impose a set of conditions on the system response. The simultaneous occurence of events, must not result a random response. A predetermined action is necessarily decided. Evidently, a successful system design starts by depicting the functional specifications into a suitable functional model. It is'nt enough to have a clear model, but it must be also free of problems. The possible problems can be detected and avoided by analysing the resulting model. The modeling tool which has a set of powerful analysis rules, is the Petri net. The Petri net structure proved effecient use in system modeling. The chosen application,on the modeling procedure presented in the paper, is a double runway control system. A hierarchical construction of the model is illustrated with detailed explain-ation. This modeling procedure can be similarly applied to any other system.