Recently, terahertz ray imaging has emerged as one of the most promising new NDE techniques, and new systems are being developed for applications. In this work, we conducted system calibration on a new time‐domain spectroscopy system, and then utilized this system to characterize glass‐fiber composite plates and polyimide resin disks. Extensive experimental measurements in thru‐transmission mode were made to map out the T‐ray beam pattern in free space as well as to scan these two test materials. Material properties such as index of fraction and absorption coefficient are of the primary interest. Both results were shown in good agreement with known data. Using these characterized material properties, we also demonstrated accurate modeling of the T‐ray signal propagating through the polyimide resin disk.