Due to their attractive performances, inorganic scintillators such as NaI(Tl) detectors are widely used in the field of gamma-ray spectrometry where knowledge of detector efficiency is a key factor for successful quantitative  ray analysis. Monte Carlo simulations using MCNP code have been performed in order to evaluate absolute efficiency of NaI detectors. Two mathematical models were constructed for the NaI crystal: 7.62cm x 7.62cm cylindrical NaI and 7.62cm diameter spherical NaI. Detector efficiency was calculated for both NaI detectors in the photon energy range from 60 keV to 2750 keV. Simulation results were compared to experimental efficiency data.