The accumulated plastic strain of subgrade soils under repeated traffic loading is an important consideration in designing pavements that resist rutting. This study presents a laboratory evaluation of the plastic strain behavior of a cohesive soil using repeated load triaxial tests. The stress-strain response of the soil under repeated loading was determined at various deviator stress levels and carefully examined. The accumulated plastic strain and the plastic strain rate were used to characterize the shakedown behavior of a cohesive soil. Approaches to determining the critical stress that defines the boundary between stable and unstable conditions under repeated loading were developed. An equation for predicting the critical stress with varying water contents was induced from the presented data. In addition, a correlation between the accumulated plastic strain and the number of toad repetitions was established with a power law model. Cohesive subgrade at high water contents was found to be liable to develop excessive plastic strain under repeated loading.