To reveal distribution of slab thermal efficiency (STE) and its bottlenecks, a slab region thermal efficiency (SRTE) model and an STE model were established based on the energy apportionment model of a reheating furnace in this paper. The bottleneck index of the slab thermal efficiency (BISTE), which could be used to assess the influence of SRTE on STE in a particular region, was proposed. First, the regional energy balance equation was listed based on reheating furnace region division. Next, the SRTE and STE models were established. Second, the bottleneck of slab thermal efficiency (BSTE) was achieved through a partial correlation analysis (PCA) of billet samples, which were obtained according to the difference between billet loading temperature and its residence time in the reheating furnace. Next, the BISTE was advanced to accurately determine the BSTE. Finally, several suggestions or measures, that could improve SRTE, were proposed. The case study has demonstrated the validation of these models, and the BISTE was 42% (Preheating), 19% (Heating II), 18% (Soaking), 11% (Heating I) and 10% (Preheating & Heating), respectively. Therefore, the preheating zone is the key region used to improve STE. (C) 2018 Elsevier Ltd. All rights reserved.