The controlled deposition of thin films on a variety of substrates is a fundamental step in many technological applications, being of great importance to know their thickness. The thickness of a film with known composition, can be determined by using the intensity attenuation of a monochromatic X-Ray beam which traverses the film. In this work, we propose to use a SEM-EDX system to detect the characteristic X-Rays generated by a specially designed target, and that traverse a metal film deposited on a thin polymeric substrate. The target is a Cu film with 800 nm of thickness, deposited on a Ti plate which upon the incidence of a 30 keV electron beam generates three lines of comparable intensity, well separated in energy. EDX spectrums are acquired from this target and, depending on the film thickness, it was selected one of these signals to measure its attenuation, choosing the one which gives the best measure for the particular film under study. The films under study were three Ag films deposited with different deposition times, by magnetron sputtering on transparent polyethylene sheets. The target and the sheets were mounted on a specially designed sample holder, which allows to determine the X-Ray attenuation by simple 90° rotations, in order to face the detector and acquire the non-attenuated emitted signals, the signals attenuated by the uncoated sheet and the signals attenuated by the Ag coated sheet. The film thickness determined were 0.20, 0.59 and 0.99 μm, which agrees linearly with the corresponding 5, 15 and 25 min sputtering deposition times.