One of the major problems in using Ground Penetrating Radar (GPR) for estimating pavement layer thickness is the uncertainty associated with the dielectric properties of the materials. The dielectric properties of pavements may vary significantly due to aggregate type, moisture presence, etc. Therefore, variation in the dielectric properties, which range from 3 to 15, will result in misleading thickness determination. Obtaining cores for calibration may reduce the error, but the variation in the dielectric constant along the roadway warrants the development of errors in the thickness determination. A method to determine the dielectric constant, and therefore the thickness, of the hot-mix asphalt (HMA) layer of an existing pavement using GPR was developed. Due to the different compositions and ages of the layers forming HMA in older pavements, dielectric constant estimation based on the surface reflection may not be very accurate and thus lead to wrong thickness estimations. The developed method uses a modified common midpoint technique (usually used in seismic testing) to estimate the dielectric constant, based on the reflections from a common point at the bottom of the layer. Data collected from a 27km portion of Interstate 81 (I-81) were successfully processed with this technique. Comparison between the thickness estimated by this method and that measured on cores extracted from the highway revealed a mean error of 6.8% whereas the classic time domain technique showed an average error of 12.7% for the same data.