Within the last two years the FANO factor F for Ge (Li)-detectors, which defines the theoretical limit for the resolution in the area of spectroscopy in regard to the detector, was found out experimentally several times. The values obtained for F vary between 0.7 and 0.15. In order to find out the reasons for these variations the dependence of the energy resolution on the electric field was examined at various gamma-energies for several Ge (Li)-detectors. Even at very high electric fields (several kilovolts/cm) still a field dependence especially at high gamma-energies appeared, which could be explained in terms of charge collection time. As condition for an exact determination of the FANO factor the extrapolation of the resolution to field strength infinity is set up; with this method the value of the FANO factor for Ge (Li)-detectors, used here, was estimated to 0.20 ±0.05.