Gamma ray buildup factor for two shielding materials, water (Z eff ¼ 7.42) and graphite (Z ¼ 6) for isotropic plane source normally incident within the energy range 4e10 MeV and up to 5 thickness mean free path has been studied. This research has been achieved by writing a simulation computer program in (Visual Basic language version G) and applied depending on Monte Carlo simulation method. The contribution of annihilation effect on the calculation of gamma ray dose buildup factor has been studied for both graphite and water. The results indicated that the calculated values of dose buildup for plane source, when ignoring the contribution of pair production, is less in comparison with the case when the contribution of pair production is taken into consideration. However, the contribution of Compton effect is still more predominant than the effect of annihilation within the range of energy (4e10) MeV.