Background: Polymer composites when reinforced with heavy metals in the form of micro/nano particles are efficient gamma- and X-ray shielding materials providing such advantageous features as cost-effectiveness, light-weight factor, flexibility, non-toxicity, conformability over conventional shields. Materials and Methods: In this paper the influence of weight percentages and particle sizes of bismuth and tungsten contents of reinforced composites, i.e., bismuth/rubber (Bi/R) and tungsten/rubber (W/R) shields, in their photon absorption capabilities for photon energies, ranging from 40 to 350 keV, were investigated using both MCNPX simulations and measurements. The Bi/R and W/R polymer composites were prepared by mixing rubber with different weight percentages of bismuth and tungsten powders before exposing them to ¹⁵²Eu gamma-rays. Then, the shielding efficiencies or photon transmission fractions, half-value layer (HVL) and tenth-value layer (TVL) of the samples were determined. Results: The simulation results confirmed that the particle size of the heavy element plays an important role in the shielding efficiency, especially at low photon energies. The decrease in the particle size of shielding material in each weight percentage improved the radiation shielding features. Therefore, the results supported the feasibility of nano-sized composite applications for shielding against low-energy photons, especially in diagnostic studies such as mammography. Conclusions: At low photon energies, especially around 40 keV, a considerable decrease in photon flux was achieved by incorporating nano-sized heavy elements in a polymer matrix.