In this paper, we compare the performance of dispatching rules in a real-life job shop environment and provide guidance for schedulers to determine effective dispatching rules for this type of systems. We consider a total of 20 dispatching rules, that range from some previously developed rules to some recently developed sophisticated rules such as process time plus work in next queue plus negative slack (PT + WINQ + SL), multi factor rule, and bottleneck dynamics. The performance measures examined are average weighted tardiness and proportion of tardy jobs. Discrete event simulation model based on ARENA is developed to implement the rules. Results from this study are given in detail.