Abstract: The determination rules of indentation diagonal in Vickers hardness test in American standard and national standard were compared. The national standard stipulated that the difference between the two diagonals should not exceed 5% of the average value. The American standard stipulated that, if half of the length of one diagonal was greater than 5% of half of the length of the other diagonal, the surface of the sample could not be perpendicular to the axis of the indenter, while NADCAP’s audit guidance recommended that half of the length of one diagonal should not exceed 5% of other half of the length of the diagonal. In the verification process of the test results, the differences in the determination of the indentation diagonal by the above three methods were compared respectively, so as to provide reference for the Vickers hardness testers to determine the indentation diagonal.