Metal polymer composites (MPCs) are new category of advanced materials whose effectiveness in the field of radiation protection has been confirmed experimentally and theoretically. By using MCNP code, this paper describes a modified model to study effect of size and proportion of tungsten (W) particles on shielding properties of light density polyethylene (LDPE). Indeed, LDPE/W composite is composed of LDPE as a background in which W particles in different sizes and proportions are distributed. Accordingly, we designed a structure containing W particles in sizes of ۱۰۰ nm, ۱ μm, ۱۰ μm, ۱۰۰ μm and proportions of ۱, ۵, ۱۰, ۱۵, ۲۰, ۲۵ wt% inside LDPE matrix. Shielding properties of LDPE reinforced with W particles in terms of size and proportion of W have been discussed. The results showed that transmission factor increases sharply in low energy region up to the maximum, then follows a slow descending trend. It was found that the values of transmission factor decrease with an increase in the proportion of W, as it was observed in all W sizes. Besides, the lower the filler proportion is, the more significant the size effect occurs. To find more, we introduced a formula to compute relative reduction of transmission (RRT) from ۱ to ۲۵ wt% at each energy point. The maximum value of RRT occurred at ۰.۰۵ MeV which enhanced up to ۸۹% while W proportion increased. Besides, the maximum buildup factor was observed at ۰.۱ MeV which is lower in LDPE/۲۵ wt% W composites compared to ۱ wt% ones. In other word, increasing W proportion up to ۲۵ wt% resulted in decreasing the maximum buildup factors and this reduction was more notable in greater W sizes. Furthermore, it was proved that the filler proportion plays more effective role than the filler size in attenuating gamma rays