A major problem in using ground penetrating radar (GPR) for estimating pavement layer thickness is assuming the dielectric properties of that layer. Pavement dielectric properties may vary significantly due to aggregate type, moisture presence, and other conditions. Therefore, uncertainties in the dielectric constant, which may vary 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 often leads to errors in the thickness determination. A method was developed to determine the dielectric constant, and therefore the thickness, of the hot-mix asphalt (HMA) layer of a pavement using GPR. Because of the different compositions and ages of the layers forming HMA in older pavements, dielectric constant estimation based on the surface reflection may not be accurate and may 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 were collected from a 27-km portion of Interstate 81 and 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%.