Performance of unpaved granular bases relies on the shear strength and stiffness of granular particles. These two mechanical properties depend on shape, size, and gradation of the particles. This paper presents a micromechanical study on the influence of particle gradation on the response of unreinforced and geogrid-reinforced granular bases under cyclic loading. Commercial software, Particle Flow Code (PFC) 2D based on the discrete element method (DEM), was used for this purpose. Different biaxial assemblies of uniform particles and well-graded particles were sheared at low confining stresses for their stress-strain relationships and friction angles. The geogrid was simulated using one layer of bonded particles with different sizes to mimic its grid structures. Two base courses consisting of uniform particles and well-graded particles, which were subjected to a 5kN cyclic ramp load at a frequency of 0.76 Hz using a wheel, were modeled. The DEM study showed that the assembly of well-graded particles had a higher angle of internal friction than the assembly of uniform particles. Further, the addition of the geogrid reduced the deformation after each load cycle.