Ultrasonic is one of the most common uses of non-destructive evaluation technique. It could detect flaws inside the structure and on the surface such as voids, holes and cracks. In this investigation, a 304 steel block with a small hole inside the structure was fabricated. The radius of the hole was evaluated by the pulse-echo ultrasonic technique. In this work, 2.25MHz, 5MHz and 10MHz of incident waves were employed to detect the radius of the hole inside the structure. The errors of a hole with 2.5 mm radius measured by 5 MHz and 10MHz longitudinal wave were 1.15% and 6.6%, respectively. Experimental results showed that the accuracy of flaw sizing detection by ultrasonic technique depends on the frequency of the incident wave.