The CONWIP production control system has received a great deal of attention from researchers recently. In this paper, we introduce a method to determine the card number of the CONWIP system for a production line with a bottleneck. The simulation study compares the CONWIP system and the original control system for the four situations in a cold rolling plant. Simulation results show that the CONWIP production control system is very elcient for the production and inventory control of semi-continuous manufacturing, such as that found in the steel rolling plant. It can greatly reduce the work-in-process (WIP), decrease the average inventory and average inventory costs, and guarantee a higher throughput rate and facility utilization.