Abstract: C/SiC composites drilled with the blind holes of various diameters and various depths were prepared in order to simulate defects of air voids or delaminations under the surface of the material. Thermographic technique，due to its high sensitivity to thermal radiation ability of different materials，was utilized to detect sizes and locations of the defects in specimens. The detection resolution，advantages and disadvantages of thermographic technique were compared with those of X-ray radiography and X-ray CT. The results indicate that thermographic technique is able to detect the geomet ries，dimensions and depths of the defects in C/SiC composite. For the blind holes with the same depth，the measurement error of diameter and depth increases as the diameters decrease；for the blind holes with the same diameter，the measurement error of diameter increases as the depths decrease，but the measurement error of depths reduces. The thermographic quantitative measurement of blind holes in C/SiC composites has a lower limit.