Abstract: Ultrasonic testing (UT) has been proven to be a high-accuracy and high-reliability Nondestructive testing (NDT) method for determining defect in composite structures, such as its size, location, depth etc. While enjoying widespread application, UT is now considered to be interpreted from the view point of uncertainty. This paper aims to evaluate the uncertainty in UT quantitative assessments for composites defect size. Ultrasonic pulse-echo method was performed on composite test block made of CFRP with flat-bottom holes of different diameters. The geometry of defect was determined by referring to bottom signal amplitude, which would attenuate by 6 dB. Various factors, including the test block, scanning method, instrument parameters, as well as the repeatability, have been taken into consideration. This paper also covers the methods and steps of the evaluation of uncertainty in defect determination by UT.