A NaI detector is a type of scintillation detector used to detect gamma rays. NaI detectors are widely used in gamma-ray spectroscopy, which has been used in this study for multiple radioactive sources. NaI detectors are also relatively inexpensive and easy to use, which makes them a popular choice for many applications in nuclear physics, nuclear medicine, and environmental monitoring. MCNP stands for Monte Carlo N-Particle, which is a general-purpose Monte Carlo code used for simulating the transport of particles such as neutrons, photons, electrons, and other particles through complex geometries. It is widely used in the fields of nuclear engineering, health physics, radiation protection, medical physics, and many others. In this research, an experiment was performed for the characterization of NAI(TI) detector. To compare the experimental results, The MCNP6 code was used to characterize a 1.5″ x 1.5″ NaI(Tl) scintillation detector by determining its energy resolution and full energy peak (FEP) efficiency curves in the energy range from 59.5 Kev to 1115.5 KeV. Simulation results match experimental behavior with minimal differences in energy resolution and efficiency curves, ranging from 0.12% to 8.35% and 2.51% to 5.13%, respectively.