Suffusion is the erosion of small particles through the skeleton of coarse grains. Soils susceptible to suffusion are described as internally unstable. In this study three dimensional discrete element modeling is employed to investigate internal instability in soils. The simulation is achieved by assessing contacts distributions, forces analysis and transmitted stresses between particles. Three types of gradations have been selected for the analyses: linear, concave upward and gap-graded. Observations of mechanical coordination number and contact distributions during isotropic compression show that the number of fine particles with low connectivity is comparatively higher for gap-graded and concave upward gradations. The evolution of contact force networks confirms that internally stable soils have a relatively homogeneous network of contact forces compared to internally unstable soil. Force distribution analyses reflect higher percent of weak contacts and low connectivity for fine particles in internal instability. In addition four commonly used internal instability assessment criteria were contrasted with micromechanical parameters, and findings revealed reasonable compliance between stability indices and micromechanical measures. Finally the stress reduction factor of the soils is calculated, confirming previous experimental and numerical studies that a is higher for internally stable soils. (C) 2015 Elsevier B.V. All rights reserved.