The results of laboratory test play an essential role for physicians to diagnose medical conditions and lead time is the major indicator of the performance. Due to the critical role of medical laboratory, increasing number of hospitals adopt total laboratory automation (TLA) to improve the performance. Although the average lead time has a significant improvement, lots of work in process (WIP) cause by specimens piled up with 40-count at the centrifuge module and batch mode at the DXC module in some peak hours. Therefore, the use of lean value stream mapping (VSM), constant work-in-process (CONWIP), and simulation were adopted. The decision variables of the upper limit WIP level, two break points for scheduling rules, and the batching time will be decided by simulation optimization. The methodology is compared with the original system in a real practical case. The results show that 74.36 % improvement in the service level was achieved.