Abstract: The interaction of X ray between Al plating and U matrix materials, the effect of U matrix's radioactivity to measurement of Al thickness has been discussed. A mathematical model of the relationship between intensity and plating thickness has been advanced, as well as in the same model, U M and U L ray are selected to measure thin and thick sample, respectively. Nondestructive testing method of Al plating thickness on U matrix has been found by measurement microarea on the working sample using collimating technology, and carrying out Least Square Regressing Analysis for the result of measurement.