Relative intensities of secondary X-rays excited by 241Am and 238Pu sources in Fe and Cu foils with different thickness have been measured by a Si（Li） detector in the incidence direction. The experimental results indicate that:1) intensity of secondary X-rays is raised with the increase of the foil’s thickness which is ranged from 5 to 100 μm; 2) there are more characteristic X-rays excited in the higher Z materials; and 3) intensity of secondary X-rays is related to activity

and energy of the exciting source and the way of the foil’s formation. Finally

the mechanism for the experimental results are briefly discussed.