The stiffness modulus and density of pavement subgrade contribute significantly to the long-term performance of a pavement structure. Subgrade functions primarily as a support for road pavement structures. Poor performance of the pavement structure is often a result of a lack of quality control during the construction of the subgrade layer. This paper presents a case study in which a falling weight deflectometer (FWD) test was used to evaluate whether the subgrade layer had achieved the required design stiffness modulus and density during construction. The characteristics of the FWD deflection basins were analyzed and the stiffness modulus was back-calculated using the CIRCLY5 pavement analysis program. The problems associated with FWD testing directly on subgrade are discussed, and an appropriate test load is proposed. Deflection-based models are developed by relating the FWD center deflection with the in situ stiffness modulus and density of the subgrade layer. A dynamic cone penetrometer test was carried out to determine the in situ stiffness modulus, and the results are compared with the back-calculated stiffness from CIRCLY5.