The design and the analysis of flexible pavement systems depend on soil layer characterization, traffic loads, and number of passes. The current AASHTO design method for flexible pavements uses resilient characteristics of subsoils to characterize and determine the structural support of each layer and to design the thickness of the layers. This moduli property, however, does not fully account for the plastic strain or rutting potentials of subsoils, as in the cases in which silt and mixed soils undergo high plastic deformations but possess high resilient properties. A study was initiated to establish a test procedure to use a repeated load triaxial device to measure plastic strain potentials of subgrade soils. Laboratory-compacted soil specimens were subjected to a repeated deviatoric load, determined as a percentage of static deviatoric load at failure under un-consolidated undrained conditions. The plastic strains were monitored during 10,000 repeated load cycles, and the accumulated plastic deformations were determined. The test procedure and test results conducted on two types of soils, a coarse sand and silty sand, are presented. Effects of soil type, compaction moisture content, dry unit weight, confining pressure, and deviatoric stresses on the plastic strains were addressed.