Radiation measurement is vital in radiation physics as its possible hazardous effect on human health should be known. Although radiation may be measured using different kinds of detector types, the NaI(Tl) crystal is one of the most commonly used detector types. This is due to its advantages such as low cost, resistance to thermal effects, and weather conditions. In the use of a gamma spectrometer system, the performance of the detector system is very important in order to determine absolute radiation values as the variation of measurement may affect health due to the radiation. In this work, the performance of 3 '' x 3 '' NaI(Tl) detector has been determined by obtaining some parameters such as response function, resolution, energy spectrum, the figure of merit. Those parameters have been measured using Na-22, Cs-137, and Co-60 radioactive sources. The system was also modeled by using FLUKA code.