The dependence of the HPGe-detector efficiency on its intrinsic and geometrical properties such as energy resolution, electronic noise, charge production statistics and effect of the rise time is considered. This study also includes the detector's sensitive volume. Efficiencies of a 0.55 cm3 planar and 46.3, 56.4 and 62.3 cm3 active volume of coaxial HPGe-detectors have been estimated for γ-ray energies from 50 to 1000 keV only, for a point source. Agreement has been found between the present calculated and published experimental efficiencies.