Subgrade is one of the most important parts of highway structures, and has a great impact on pavement performance and traffic safety. The 1993 AASHTO (Association of State Highway and Transportation Officials) Design Guide introduced resilient modulus (MR) as an important subgrade material property (AASHTO, 1993). In the new AASHTO mechanisticempirical design guide (NCHRP, 2004), MR also plays a major role in representing the properties of the materials in various pavement layers. It is well known that the moisture content has an important impact on the MR of subgrade soil. Relationships between stress state, water content or matric suction, and resilient modulus have been established by previous studies, and different adjustment methods have been proposed. Li and Selig developed a formula to estimate the MR using water content and dry density as independent variables (Li & Selig 1994). Drumm et al. proposed that the value of MR decreased significantly with the increase of water content for specific soils (Drumm, Reeves, & Madgett 1997). Recently, Khoury found that the MR-moisture relationship is hysteretic and is vital in predicting the response of pavement structure because of changes in climate (Khoury 2009). In fact, the moisture of subgrade does not keep stable but fluctuates seasonally. More importantly, the moisture fluctuation beneath the pavement is not completely dry to fully saturated, but in a certain water content
range near the Optimum Moisture Content (OMC). Some scholars used the strength reduction factor of the freezing-melting cycle to correct the influence on subgrade resilient modulus. Similarly, it is worth investigating whether the limited range of moisture fluctuation causes MR reduction, and how the impact of moisture fluctuation on MR. is best described. Therefore, in this paper, the moisture fluctuation simulation method was introduced, and then the impact of moisture content fluctuation on the MR of compacted clay was investigated by laboratory experiment.