Three dimensional discrete element modeling is employed to investigate micromechanical behavior of internally unstable/stable gap-graded soils under isotropic compression. Two gap-graded soils, one internally stable and the other unstable, are modeled by assemblies of spherical particles and the micromechanical parameters in stress transmission are inspected. The variation of coordination number and contacts per particle during isotropic compression suggests higher coordination number for internally stable soil. Mechanical coordination number reveals that floating particles with low number of contacts are more frequent in the internally unstable soil. The evolution of contact force networks during compression shows that internal instability corresponds to a more heterogeneous contact force network. The results confirm the hypothesis that loose particles nest within pores of the primary fabric (coarser fraction) which is preponderant in transferring stresses. Finally, the variation of stress reduction factor (alpha) with confining pressure and void ratio, confirms previous experimental and numerical studies that a is higher for internally stable soil. (C) 2013 Elsevier B.V. All rights reserved.