In a flexible pavement, the base course is an essential component in that it supports the surface course and distributes traffic loads to the subbase and subgrade. Asphalt treatment is used on granular base course material to enhance the material's properties and overcome its deficiencies. Due to its lower binder content and lower-quality granular material, asphalt treated base (ATB) exhibits a stress-strain relationship unlike either regular granular material or hot mix asphalt. Stress-strain relationship is the primary fundamental material property used for mechanistic-based pavement structural analysis and design. However, current guidance for choosing the parameter that characterizes the stress-strain relationship of ATB in pavement design is not clearly provided. This paper presents a study in which the stress-strain relationship of differing base materials, including three ATBs, a mixture of reclaimed asphalt pavement and granular material, and a typical granular base material, is compared and analyzed based on data measured from repeated triaxial compressive tests. The resilient modulus (MR) was found to be capable of representing the stress-strain relationship of all materials selected. Further, the MR of selected ATBs was calculated at different stress states represented by bulk stress and octahedral shear stress to investigate and compare the stress-dependent property of the MR of selected materials. The stress-dependent property mechanism for each type of ATB was analyzed. The effects of stress-dependent MR on pavement performance were evaluated. It was found that pavement distress at the end of the design life could be either underestimated or overestimated if the representative value of MR is not properly selected.