Abstract: Different diameters and depths of blind holes were drilled in the samples of 3D needled C/SiC composites to simulate air void defects in this paper. Thermal imaging testing was used to detect the blind hole defects, and then the dimension and location information of the defects were obtained. Two methods, i.e. peak temperature-contrast method and logarithmic peak second-derivative method, were used to measure the defect depths, and the errors of these measured results were discussed. The results indicate that both of the two methods can be used to successfully measure the depth of the blind holes, but the measured error for the two methods is different. The error of peak temperature-contrast method is less than the other method when the ratio of diameter to depth ranges from 1 to 4.4. Though the error of logarithmic peak second-derivative method is a bit larger than the other, this method can be used for wider ratios of diameter to depth, and the error of this method decreases as the ratio of diameter to depth increases.