A series of experiments on asphalt pavement specimens with 1-GHz GPR system was conducted in the Connecticut Advanced Pavement (CAP) Laboratory. The objectives of these experiments are as follows. (1) Determining the dielectric property of the asphalt specimens; and (2) Correlating electromagnetic (EM) properties with void ratio and asphalt binder content ratio of asphalt pavement specimens. Results of this study can be used as the baseline to calibrate GPR field surveys for pavement assessment, as well as an indirect means to monitor the compaction process. The travel time for direct and reflected phases are the fundamental information for computing EM wave velocity. Supplemented by pre-existing information on geometry, void ratio, composition measured by other means, we were able to compute the dielectric constant of 30 pavement specimens. The major conclusions from these experiments are as follows. (1) In general EM wave velocity is highest in dry conditions, intermediate in frozen, and lowest in watersaturated conditions; Correspondingly, the dielectric constant is smallest in dry conditions, intermediate in frozen, and highest in water-saturated conditions. (2) EM wave velocity increases slightly with the increase of void ratio for dry samples. In contrast, it decreases significantly with a void ratio increase in water-saturated conditions. Correspondingly, the dielectric constant decreases noticeably with an increasing void ratio in dry conditions, and increases appreciably in saturated conditions. (3) The changes of EM velocity and dielectric constant for dry and saturated conditions can be predicted by the effective medium theory for porous media. (4) When the dielectric constant of the pore material is taken as the value of fresh water ice, the change of EM velocity and dielectric constant for the asphalt pavement specimens in frozen conditions cannot be correctly predicted with the effective medium theory. This implies that the pore water was not completely frozen when GPR measurement was taken. There are no significant changes in dielectric constant for dry and frozen conditions. (5) Variations in EM velocity and dielectric constant with asphalt binder ratio imply that a low asphalt ratio corresponds to a high void ratio so that in the lower end of the asphalt ratio, EM velocity has maximum fluctuation among the different conditions.