A 3D unsteady numerical simulation of a reheating furnace was performed to obtain the optimal slab residence time. Too long residence time decrease the efficiency of the reheating furnace, whereas too short residence time cannot satisfy the required heating quality of a slab. The total five cases of residence times - 6032 s, 6496 s, 6960 s, 7424 s and 7888 s - were investigated for the optimum residence time analysis with the two slab requirements, those of emission temperature and uniformity. In this study, the slab emission temperature should be in the range between 1373 K and 1573 K. The skid mark severity of an emitted slab should be lower than 50K to satisfy the uniformity requirement. The numerical analysis was done for the identical geometry and operating condition of the reheating furnace using FLUENT. Slabs were assumed to move very quickly that it took no time for them to move next positions. The quick movements of slabs were processed with the own developed User-Defined Function program. Among the five cases of residence times, the residence time of 7427 s turned out to be most efficient. (C) 2012 Elsevier Ltd. All rights reserved.