Generally, queueing networks, networks, chains and simulation techniques are among the most widely used evaluation tools to study computer systems, manufacturing systems etc. But, these tools have some assumptions and difficulties. So, this thesis presents a method for manufacturing system using Petri nets. Petri nets are a graphical and mathmatical modeling tool and are promising tool for describing and studying systems that are characterized as being concurrent asychronous, dynamic, distributed and parrallel. It designs Petri net model for processing time of manufacturing system And it applied to a job shop system composed of 8 machines and 4 different job types.