Unbound granular materials (UGMs) are utilized in flexible pavements to serve mainly as subgrade materials, offering the necessary support for the imposed traffic loads. Although the likelihood of subgrade strength failure under traffic loads is minimal, its permanent deformation is more critical. The repeated load triaxial (RLT) test is a widely employed method for assessing the accumulated permanent deformation in unbound granular materials (UGMs) in their lifetime. Nevertheless, there have been challenges in establishing a direct correlation between the results obtained from RLT tests and the actual deformation measured in full-scale pavements. The primary reason for the disparity is the incapability of RLT-based designs to account for the rotation of principal stresses imposed by the application of shear stress coming from the movement of vehicles. In order to examine how the rotation of principal stress affects the formation of plastic strain in UGMs, various experiments were conducted in the present study. These experiments include different levels of cyclic stress ratio (CSR) and cyclic shear stress ratio (CSSR) using a hollow cylinder apparatus (HCA). The results obtained from the HCA tests were compared to those obtained from the RLT, and it was discovered that incorporating the principal stress rotation significantly amplified the permanent deformation of the unbound granular materials (UGMs). This indicates that incorporating the influence of principal stress rotation is essential for ensuring a proper and accurate design that can effectively address the deformation behavior of the pavement layers.