This paper proposes a work-in-process (WIP) allocation approach to improve on-time transfers of a unified automated material handling system (AMHS) in a semiconductor fabrication facility (FAB). The real-time operation and control of the complex AMHS often cause delayed transfers of wafers to manufacturing machines and result in production loss. We introduce transfer-due-dates from storages to manufacturing machines to enhance the on-time transfers in the AMHS and develop a mathematical model to allocate the WIP to the optimized storage locations based on transfer-due-date. The simulation results demonstrate that the machine utilization as well as the AMHS capability for on-time transfers can be improved by assigning the wafers’ interim storage locations more appropriately.