According to the needs of calibrating the probing size error, a method based on the standard balls for the calibration of industrial CT probing size error  is proposed and factors which cause the probing size  error are detailly analyzed. Firstly, it calibrates the diameter by the length-measuring machine of National Institute of Metrology, then calibrates the industrial CT probing  size error by these balls. The experimental results show that these standard balls can be calibrated accurately, and high quality image with lower artifacts can be acquired. And this kind of standards can meet the calibration needs of probing  size error of industrial CT system. Error sources in imaging process, which influence the probing  size error are discussed and analyzed. It is of great significance to establish calibration and traceability method for industrial CT system.