Despite the claims by software vendors on positive values of an integrated MES and ERP system, there has been no systematic study conducted to assess and evaluate the impact of such an integrated system on shop floor operations. This paper presents a simulation study to evaluate the impact of the MES integration with the ERP system on production lead times. First, we describe a methodology of Using a discrete event computer simulation to address an inherent problem of the Enterprise Resource Planning (ERP) system of handling uncertainties and unexpected events. Then, simulation study results comparing the performances of a manufacturing system with MES and a manufacturing system without MES are presented. The evaluation metric used in this simulation is the production lead time. However, the results obtained in this study can be expanded to more general situations with different evaluation metrics.