Parallel computer systems are well known to be very complex and their design involves difficult hardware/software tradeoffs. For such complex systems an early performance evaluation is crucial, especially when several designs of the same system are to be compared. The design of parallel complex systems, which generally have real time requirements, consist of concurrent processes and imply special hardware, can only be done using an overall comprehensive method, In recent years many new techniques, methods and tools have been used in the attempt to overcome the design problems of complex systems, A global understanding of the design process is the key factor in defining a valid new approach that will enable accurate performance evaluation of the system during the design phase, The paper presents a design method and a new user-friendly software tool called APSPS (Asynchronous Process Simulator for Parallel Systems), The simulator, APSPS, is built on a PC platform with a graphical interface, Several features such as ease of understanding, ease of design, abstraction and reliability were kept in mind when developing the proposed simulator, The simulator was developed using object oriented concepts in the C++ language which provides the features in mind, and enables the creation of a general simulator.