The modulus of the asphalt layer in a pavement structure subjected to vehicular loading is different from that subjected to falling weight deflectometer (FWD) loading. A comprehensive approach is developed in this study to investigate the relationships between asphalt layer moduli under the two loading conditions. Two full-scale asphalt pavements named flexible pavement and semirigid pavement were constructed. The structural responses of these two pavements under vehicular and FWD loadings were measured. The modulus master curves of asphalt layers in field pavements were developed based on the measured responses and the finite-element model. The developed master curves were found to be able to accurately predict asphalt layer moduli under either vehicular or FWD loading. Based on the obtained master curves, the ratios of asphalt layer moduli under vehicular loading to those under FWD loading were calculated under various conditions. The calculated ratios are mostly lower than 1.0 in a wide range of vehicular speeds, regardless of temperatures. The ratio increases with rising vehicular speed, but does not show a consistent trend with pavement temperature. In particular, the ratios are noticeably lower at intermediate temperatures than those at either high or low temperatures. Hence, the use of FWD to determine the stiffness moduli of asphalt layers may significantly overestimate their true stiffness in typical in-service conditions. The methods and findings in this study may provide a useful reference to correlate asphalt layer moduli between vehicular loading and FWD loading.