Aggregate breakage in unbound pavement layers can lead to pavement distresses that affect their functionality and service life. Thus understanding the mechanics and clarifying the factors affecting materials breakage resistance are important for ensuring adequate performance of these layers. In this study, aggregate breakage in unbound granular materials (UGM) is investigated experimentally and numerically. Experimentally, aggregate breakage under uniaxial compression is examined for two UGMs prepared with the same aggregate type but different gradations. To capture the experimentally observed influence of gradation and load magnitude on aggregate breakage, a Discrete Element Method (DEM) model was developed, based on granular mechanics particle contact and failure laws. A simple procedure to identify the contact and failure law parameters from experiments is proposed. With those parameters, the model’s capability of capturing the effect of gradation and loading on the aggregate breakage in UGM is evaluated. Based on comparison with experimental findings, it is shown that the model can capture macro-scale properties of UGM, such as its deformation response under uniaxial compression, as well as the amount of aggregate breakage in the material.