In this research, discrete-event simulation is used to study the I/O drawer test process in a high-end server manufacturing environment in order to identify the optimal batch size of the I/O drawers to be tested per driver. High-end server manufacturing environment is characterized by fast lead time and lengthy build process. Therefore, main components of the server, such as I/O drawers, are tested ahead of time (fabrication test) and stored in inventory as tested parts to be ready for prompt fulfilment of the customer orders. In this research, a simulation model is developed for the I/O drawer test process with a focus on batching the I/O drawers on testing. Different scenarios for the batch size are considered and a statistical comparison is performed against the current scenario used by the I/O drawer Fab test operators. Unlike the "one-piece flow" lean concept which encourages small batch size (even one), the results show significant savings in cycle time and energy consumption when the batch size is increased. This is attributed to the lengthy setup time of the I/O drawer testing process as using small batch sizes requires very short set-up time. The optimal batch size scenario results in cycle time savings by 20% which is equivalent to 8116 hours per year. Other savings include: electrical energy and less consumption of chilled water for the cooling units.