Low effective stress in the fine fraction of internally unstable materials is the key mechanism leading to the internal instability. In the present study, the effects of stress state and fine fraction on the stress transmission in gap-graded internally unstable granular mixtures are investigated at microscale by discrete element method. It is found that the fine fraction plays a more dominant role than the stress state of soil in the stress transmission. The sensitive range of fine fraction with respect to number of contact (25-35%) is relatively lower than that of contact force (30-40%). The influence of stress state is only significant within a specific range of fine fraction, and the contributions of isotropic pressure and stress ratio to stress state influence are different. The internally unstable materials with an interactive-overfilled load-bearing fabric could switch to internally stable by increasing the isotropic pressure. The influence of stress ratio is associated with the magnitude of isotropic pressure. (C) 2020 Elsevier B.V. All rights reserved.