Abstract: It was proved that X-ray apparatus is not measuring instruments by the basic and general theory of metrology. X-ray apparatus is not regarded as measuring instruments in the national code of metrology. It is misleading that the X-ray apparatus is confused with X-ray radiography and some technical specifications of X-ray apparatus are described as the characteristics of measuring instruments by subjective idea in the inspection procedures of JJG 40—2001. The mistakes not only infract the basic theory of metrology,but also depart from the X-ray radiographic testing process.The conclusion is only that X-ray apparatus is not measuring instruments which should be reach out by the theory of metrology and the radiographic practice.