In this thesis, gamma–rays spectra measured by a NaI(Tl) 3 in ×5 in ×16 in detector were analyzed. For this, a series of radioactive sources emitting gamma-ray of known energies together with a 137 Cs source, were measured with the detector. To determine the location of peaks in the measured spectra APTEC software was used. Determination of peak channel in the spectrum of 137 Cs source showed that the peaks shift to lower channel by time. To calibrate the detector properly, the peaks of other sources were shifted with a linear relationship using the peak of 137 Cs source in each spectrum and the spectrum of 137 Cs which was measured alone (the first spectrum measured). Then, the detector was calibrated using peaks transmission. To remove background under the peaks and determine FWHM of detector, the spectrum of each source was determined with the MCNP code without Gaussian distribution for the peaks and was subtracted from experimental spectrum. Fitting a Gaussian function on pure peaks, FWHM in the energies were computed. FWHM defined function in MCNP code was fitted on obtained FWHM for several energies, then FWHM was obtained for energy range of 0.06 MeV to 2.6 MeV. Next, to determine counts related to each energy in the spectrum of sources that emit more than one gamma and overlap in the experimental spectrum, the experimental spectrum of these sources were built. For this, the energy spectra of the sources were first simulated with MCNP code using Gaussian Energy Broadening (GEB). Then simulated spectra were summed and normalized so that measured spectra were made. Making experimental spectrum by use of simulated spectra allow to calculate the number of counts of each energy. This method is also used to determine the counts related to any peak in a compound spectrum. This was done for both sources of 60 Co and 133 Ba. . Keyword : NaI(Tl) detector, FWHM, simulation of spectra, gamma-ray, MCNP code.