This study explores the effects of the size ratio and fines content on the internal stability of binary mixtures using the discrete element method. First, microscopic characteristics, including the particle contact, stress and local structure, are carefully examined. Then, internal stability evaluations of binary mixtures are carried out using two existing stability criteria, which are not consistent with the microscopic characteristics. Lastly, a new stability criterion for evaluating the internal stability of binary mixtures is proposed, which is found to be acceptable. (C) 2019 Elsevier B.V. All rights reserved.