The evaluation of integrity of critical structures and parts is of vital importance for the maintenance of quality and safety. Quantitative nondestructive evaluation (QNDE) techniques provide the means to detect a flaw and to determine its location, size, shape and orientation. Ultrasonic technique is one of such commonly used ones. Comprehensive discussions of ultrasonic QNDE methods can be found in recent review papers by Thompson and Thompson [1] and a recent symposium volume edited by Datta et al. [2].