Abstarct: Lead and lead-baxsed materials are commonly used as a shield against X- and gamma- rays in the environments exposed to radiation. Toxicity of this material caused irreversible effects on the people and environment. Therefore, an increasing efforts have been done to introduce a suitable replacement for lead. Although mextals such as tungsten and bismuth are considered very good substituents for leas due to their high atomic number, introducing other parameters such as flexibility, workability, chemical stability, mechanical strength, thermal stability, low cost manufacturing, and low weight have been attracted attention to a new category of materials called composites in a great number of studies. mextal-polymer composites (MPCs) are a new category of advanced materials whose effectiveness in the field of radiation protection has been proven both experimentally and theoretically. Using the MCNPX Monte Carlo code, an improved method is presented for modeling the polymer matrix reinforced with particles that is able to investigate the simultaneous effect of particle size and proportion on improving the ability of these materials to attenuate X-ray and gamma radiation so that structure homogeneity is preserved. mextal particles and mextal oxide were used to strengthen the polymer structure. Such a product has a high potential for use as a photon shield. mextals including lead (Pb), tungsten (W), zinc (Zn), and titanium (Ti) and their oxides including lead monoxide (PbO), tungsten trioxide (WO3), zinc monoxide (ZnO), and titanium dioxide (TiO2) was used as a macrostructure as well as microparticles and nanoparticles to reinforce polymer matrix composites. In order to find the proper geometry and to assure the variance reduction methods used, the buildup factors were calculated for the mextals and mextal oxides microstructures. According to the results, point detectors were used to increase the accuracy of the simulation and reduce the runtime of the program. Next, modeling for light density polyethylene (LDPE) structure reinforced with Pb, W, Zn, and Ti particles in sizes of 100 µm-100 nm and proportions of 1-25% wt% was done and shielding parameters including mass attenuation coefficient (μ/ρ), mean free distance (mfp), transmission factor (T), and buildup factor (B) were calculated for these structures. Using the relative difference between the amounts of nanostructures versus microstructures, the effect of particle size and proportion on the attenuation of the gamma radiation was evaluated and it was found that the effect of proportions on smaller particle sizes was less significant. As the last part of the computation work, theoretical modeling was done in a good geometry for LDPE structures reinforced with nanoparticles and microparticles of mextal oxides PbO, WO3, ZnO, and TiO2 and a comparative and comprehensive study on the shielding parameters including mass attenuation coefficient (μ/ρ), mean free distance (mfp), half-laxyer thickness (HVL), effective atomic number (Zeff), effective electron density (Ne), transmission factor (T), and buildup factor (B) were done. On the basis of the photon-matter interactions at different energy intervals, it was found that these structures have the ability to attenuate radiation by ~ 98% at low energy region. Then, the experimental study of epoxy resin structures reinforced with Pb, Zn, Ti, PbO, ZnO, and TiO2 nanoparticles for 241Am, 137Cs, and 60Co sources relying on proportions (5 and 25 wt%) and size effect (for Pb particles only) was performed. transmission factor (T), and buildup factor (B) of these structures were evaluated using area under photopeaks analysis. The results showed that Pb and Pb-baxsed structure have the best performance in attenuating gamma radiation both as a macrostructure and as microparticles and nanoparticles in the composite structure. However, the use of nanoparticles of other materials especially in the high proportions showed a good performance in protecting gamma rays. Finally, it should be noted that these materials perform remarkably well only at low energies in the investigated thicknesses (<200 mm).