Abstract: To investigate the details related with the defined maximum values of tube voltage in some radiographic testing standards, the very contents in present effective standards and new developing standards were compared and analyzed, and then the origin and changes of this specification were traced. Thus, relationship between maximum values of tube voltage and penetrated thickness was quantitatively analyzed, and correlation between maximum values of tube voltage and contrast of testing result was quantitatively calculated roughly. From the analysis, we know the curve of maximum X-ray voltage versus penetrated thickness is made according to certain math function. Maximum values on the curve are the best practice values for film radiography, however, these values are not quite applicable for digital radiographic testing. If comprehensive analysis on essential contrast and signal-to-noise in radiographic testing was carried out, maximum X-ray voltage applicable for digital radiographic testing would be prescribed.