The purpose of this study is to evaluate the externalities of the steel industry. Externalities in the steel industry refer to the impacts that steel production has on society and the environment, which are not accounted for in the final product price. One of the most robust methods of investigating this issue is the life cycle assessment (LCA) approach to evaluate the externalities of the steel industry. Our study focused on Mobarakeh Steel Company which the assessment is conducted based on the ISO 14040 standard in four steps: goal and scope definition, inventory analysis, life cycle impact assessment, and interpretation. The software SimaPro version 9.5.0.2 with the Ecoinvent database was applied with supporting ReCiPe H method, which has 18 midpoint indicators and 3 endpoint indicators. The results show the shares of mining and concentrate production process accounted for 38% of the environmental impacts while 26.6% of the impacts were related to the electric arc furnace process. The share of environmental impacts of the DRI unit was15%. In addition, three indicators, human carcinogenic toxicity, fossil resource depletion, and ozone formation in terrestrial ecosystems, were among the most influential indicators in these processes. To quantify the externalities, the cost of each of the identified environmental and social damages are evaluated based on available sources as well as the monetary value calculated for each type of damage. Accordingly, the total externality cost of producing one ton of steel sheet was approximately 846 EUR. Of this total cost, the share attributed to global warming damage was around 462 EUR, identified as the costliest environmental and social damage.