One important factor in the HPGe detectors is the efficiency that highly dependents on the geometry and absorption factors, so that when the configuration and geometry of the sample – detector is changed the detector efficiency must be re -measured. The best way of determining the efficiency of a detector is measuring the efficiency by the use of standard sources, but an easier method is to determine the efficiency by Monte Carlo simulation. In this study, the full-energy peak efficiency and dead layer thickness of an HPGe detector was obtained using the MCNPX code and experimental data. For this, we first measured gamma –ray spectra for different sources placed at various distances from the detector and stored the experimental spectra obtained. We then obtained the same spectra under similar conditions by Monte-Carlo simulation and compared with the corresponding experimental spectra. Comparison of the calculated spectra with the experimental spectra showed some differences. By making small variations in the dead layer thickness of the detector (about a few hundredths of a millimeter) in the simulation program we tried to remove these differences and in this way a dead layer of 0.57 mm was obtained for the dead layer of the detector. The full-energy peak efficiency of the detector was then obtained both by experiment and by simulation for various sources at various distances from the detector and showed good agreements.