It has been well established that the mechanical behavior of unbound aggregate materials is strongly influenced by the shape of the particles and the complete grain size distribution (i.e., gradation). This paper presents the use of an image-aided discrete element method (DEM) approach to realistically model the micromechanical interactions of aggregate particles including size and shape effects. A realistic unbound aggregate model is developed from previous studies to simulate triaxial compression tests based on the DEM approach and the innovative use of membrane elements surrounding the cylindrical aggregate specimen in the DEM model. To calibrate the DEM model, six different types of aggregates were used to prepare samples of 21 unbound aggregate blends at the same gradation and voids content but different crushed percentages. The shape properties of flatness and elongation, surface texture, and angularity were quantified by imaging then correlated to strength and permanent deformation characteristics obtained from triaxial testing. The experimental test results are utilized to determine required parameters for the discrete element model by minimizing the differences between DEM-simulated triaxial shear strength results and experimental ones. It is found that the developed DEM model is not only capable of reproducing the typical shear strength behavior of different unbound aggregate materials with varying shape properties, but also has the potential for optimizing the selection of size and shape properties of various unbound aggregates to achieve desired shear strength (or rutting resistance) and hydraulic behavior for open-graded permeable aggregate base.