In a recent work on WO3-filled epoxy composites [19], we investigated the effect of nano-sized and micro-sized WO3 filler-epoxy composites on X-ray attenuation in the X-ray tube voltage range of 22–127 kV generated by a mammography unit and a general radiography unit. The equivalent X-ray energies for the various X-ray tube voltages used were in the range of 17.5–60 keV, which conformed to our expectation since the equivalent X-ray energies for a mammography unit were 17.5, 19.6, 20.2 and 22.7 keV, which are the characteristic energies of molybdenum and rhodium, while the equivalent X-ray energy for a radiology unit is about one-third of the X-ray tube voltage used. The results showed that nano-sized WO3 was more effective than micro-sized WO3 in X-ray attenuation only in the low X-ray tube voltage range of 22–35 kV, but this size effect was not apparent at the higher X-ray operating tube voltage range of 40–120 kV [19]. Hence, the aim of this work was to verify our previous work on X-ray transmission in WO3-filled epoxy composites by using synchrotron radiations as the X-ray source for the characteristic (monochromatic) X-ray energy range of 10–40 keV. The results obtained were compared with those of previous work [19] to determine the equivalent energy range of the previous machines used (a mammography unit and a radiology unit).