Absorbed fraction of energy is defined as the ratio of absorbed energy in a material to the total emitted energy by the source. In this thesis, absorbed fraction of energy is calculated for some different geometries and materials using the computational code "MCNP". The materials were assumed to have unit-density. The gamma ray source is either a point source or a uniformly distributed source. Energy of the source is in the range of .02MeV-2.75MeV. The absorbed fraction calculated by the code, in some cases is in agreement with the results of other works. These cases are those in which the geometry is simple, for example sphere, but in other cases such as ellipsoid, the difference is larger. The reasons of difference between the results of the code and other works are investigated. Variation of absorbed fraction with source energy, geometry, material, mass of the target, point or uniformly distributed source and type of interactions have also been studied. Furthermore, the effect of surrounding environment of target on the absorbed fraction is also studied. Finally we can claim that code has the ability of investigating any complex cases in the absorbed fraction calculations.