In this paper, the new software named DETector SIMulation (DETSIM) was developed from PENELOPE code system method to simulate gamma spectra and calculating full energy peak efficiency (FEPE). The new software is using graphic user interfaces for easy-to-use purpose. As a first application post test calculations has been performed for an experiment of a mixed source ( 241 Am, 109 Cd, 57 Co, 139 Ce, 51 Cr, 113 Sn, 85 Sr, 137 Cs, 60 Co and 88 Y) located at two positions away from detector window and energy ranges between 60 keV and 2000 keV. The results showed a general agreement between the simulated efficiencies and the experimental data. The simulation results are typically 7% higher than the experimental data in the low energy range. This means that our simulation program is good enough for later studies on our HPGe spectrometer.