In this paper, the impact of the factors batch size, setup times, priority dispatching of jobs, machine failures, rework, volume, routing and product flexibility on the performance of a job shop is studied. The performance of the simulation models are measured in four dimensions: mean flow time, average work-in process, percentage throughput of the shop and the percentage of jobs meeting their due dates. The simulation results show that routing flexibility and machine selection rules have significant impact on the system performance. It is also found that the system performance starts to deteriorate when the level of routing flexibility increases. Copyright © 2010 Inderscience Enterprises Ltd.