Abstract: Ultrasonic probe with delay block is always used in the process for measurement of specimen thickness. Because of the involvement of the delay block, the traveling time of the ultrasonic wave will be increased. Moreover, the velocity of the sounds of the delay block’s material varies when the temperature changes, which could influence the accuracy of the measurement of the thickness. Circuit designed to deduct those increases is requested to get the true thickness of specimen. Employing different methods to realize this function, which can result in different effects in different environment of different temperatures, different coupling situations, and different materials of specimen, and soon on. Present paper was aiming at the concept of calibration of “zero point”, which was very important to the measurement accuracy of ultrasonic thickness gauge. At last effect of calibration by change of temperature was also discussed, the most popular calibration methods used in the ultrasonic thickness gauge were described and compared.