This paper presents partial findings from an ongoing research study at the University of Illinois aimed at laboratory characterization of North Carolina base course aggregates for permanent deformation model development and calibration. Monotonic triaxial shear strength tests were first conducted on four different aggregate types to establish the Mohr-Coulomb failure envelopes. Repeated load triaxial tests were then conducted on the aggregate materials at shear stress/strength ratios of 0.25, 0.50, and 0.75, to comparatively evaluate the permanent deformation accumulation trends, and subsequently develop improved rutting models for use in mechanistic-empirical pavement design methodologies. Experimental results clearly emphasize the importance of applied stress states, material's shear strength, and number of load repetitions as the most significant factors affecting unbound aggregate permanent deformation behavior. A new model is proposed to adequately capture the effects of applied stresses and shear stress/strength ratios in predicting permanent deformation behavior of base course aggregates. This proposed model can potentially be used to improve the rutting models currently used in AASHTO's mechanistic-empirical flexible pavement design procedure the Pavement ME software.