Abstract: The relationship of sensibility, contrast and visibilities of ray photography flaw detection was briefly introduced, as well as the relationship and importance of ray photography sensibility, grades of products photography technology and flaw evaluations grades. Some formulae were introduced for checking flaw size, derivation of the formulae and their relations, main factors affecting sensibility of flaw detection, hidden parameters in the standard. The above mentioned formulae might be useful for ray detection workers during their practical work to predict the sensibility and accuracy while considering the affecting factors.