Radiation detection has been a main interest for researchers as all kind of produced particles in atomic and subatomic physics based on the measurement systems so-called detector. Detection efficiency is one of the main parameter in detection system besides many other different parameters of the detector. The absolute efficiency of the gamma detector system will be used at Turkish Accelerator and Radiation Laboratory at Ankara (TARLA) is simulated using MCNPX code (version 2.4.0). The MCNP is the general purpose MC code that can be used for neutron, photon, electron or coupled neutron, photon, electron transport. The results have been obtained for NaI(Tl) detector system and compared with the experimental results. A good agreement was found between calculation and experiment.