NUCLEAR Safeguards Measurement Methods are developed by design characteristics as well as performance specifications for several systems of the gamma-ray assay. A NaI(Tl) gamma detector was characterized with respect to count rate performance, detector efficiency, dead time (DT), low-level discriminator, and threshold effects using low, medium, and high-energy radioactive point sources. Spectral data are collected using inorganic scintillation detectors NaI (Tl). Besides, the response functions of the NaI(T1) 2" × 2" detector were determined where detector efficiency and FWHM were measured at diverse sources using energy lines (356.02 keV of 133Ba, 661.7 keV of 137Cs and1173.2 and 1332.5 keV of 60Co) and calculated by using MCNP method. The Gaussian spectra generated by MCNP-5 code were compared with the experimental work. To validate the simulated result, the Gaussian Energy Broadening (GEB) card was used, yielding the best results. When comparing the obtained findings derived from MNCP calculations with the experimental findings, they showed a good agreement in their values.