Presented in the paper is a reservation based dispatching rule to achieve the on-time delivery in system LSI (large scale integrated circuit) semiconductor fabrication (FAB) with urgent orders. Using the proposed reservation based dispatching rule, urgent lots can be processed without waiting in a queue. It is possible to achieve the on-time delivery of urgent orders by reserving a proper tool for the next step in advance while urgent lots are being processed at the previous step. It can cause, however, tardiness of normal lots, because the proposed rule assign urgent lots first. To solve this problem, the proposed rule tries to find the best tool for the reservation in the tool group, which can minimize idle time, and the reservation rule is applied at all tools except for photolithography tools (bottleneck). $MOZART^{(R)}$ which is developed by VMS solutions are used for simulation experiments. The experimentation results show that the reservation based dispatching rule can achieve the on-time delivery of normal lots as well as urgent lots.