Geogrids have been successfully used to improve soft subgrade and reinforce weak base courses for low-volume roads by providing lateral confinement. However, uniaxial and biaxial geogrids with rectangular or square apertures cannot provide uniform resistance in all directions. A new geogrid product with triangular apertures was developed and introduced into the market to overcome this problem. The triangular-aperture geogrid has a more stable grid structure and can provide uniform resistance in all directions compared with uniaxial and biaxial geogrids. However, the performance of triangular-aperture geogrid-reinforced bases has not been well evaluated. In this study, unreinforced and triangular aperture geogrid-reinforced bases over a weak subgrade were constructed in a large geotechnical testing box (2 2.2 2 m) at the University of Kansas and tested under cyclic loading. During the tests, surface deformations and vertical stresses at the interface between the base and the subgrade were monitored. The test results showed that triangular aperture geogrids reduced permanent surface deformations and vertical stresses at the interface compared with an unreinforced base. The benefit became more pronounced when a heavier-duty geogrid was used. The backcalculations from the measured vertical stresses at the interface between base and subgrade showed that the stress distribution angle and the modulus ratio of base course to subgrade decreased with an increase in the number of cycles. The rates of reduction in the stress distribution angle and the modulus ratio for the unreinforced base were faster than those for the reinforced bases. This paper focuses on the stress analysis of the test sections under cyclic loading.