Gamma-ray spectrometry system with HPGe detector is a system with high resolution and relatively good efficiency which is used for analyzing and measuring gamma-ray spectrums emitted from different sources in various research centers and laboratories. Efficiency of an HPGe detector which is a function of geometry and energy is an important factor in determining the activity of radioactive sources. In this research the gamma-ray spectrum of a Europium source (which has a quite number of gamma-ray picks in the energy range from 121/8 to 1528 keV) has been measured by an HPGe spectrometry system for various source – detector distances. Since the activity of the source was known, the measured spectra were then used to determine the efficiency of the detector as a function of energy for various source – detector distances. In addition, the spectrometry system was simulated by Monte-Carlo method and the gamma-rays spectra were also determined by simulation method which showed good agreements with the measured data. The efficiency were then calculated by simulation method and compared with those determined by experimental method and the small differences were discussed. The efficiency results were then fitted with a polynomial and the efficiency were plotted versus energy for various source – detector distances. Regarding good agreements between the measured and the simulated results, it can be concluded that using the simulation program, we can determine the efficiency of the detector at any desired energy and any desired source – detector distance without any need for performing the experimental.