High resolution gamma-ray spectrometry is a nuclear analytical technique widely used for a determination of radionuclides in wide range of their activities in various kind of samples, from nuclear waste samples to the environmental samples and foodstuffs. For the quantitative determination this technique requires a determination of a detector efficiency with a high accuracy. In the Laboratory for radioecology of the Ruder Boskovic Institute in Zagreb, we are using Canberra’s high purity germanium detectors for this type of measurements. As the detector efficiency depends on an energy of emitted gamma rays, physical properties of materials and a sample geometry, calibration standards with same characteristics should be used for the efficiency calibration. However due to a large variety of the materials that have to be analysed, it is hard to obtain standard materials of the same characteristics. Therefore, the possibility to determine the detection efficiency in a mathematical way by detector and sample setup modeling can significantly facilitate the procedure. The description of detector and its peformances, together with the comparison of accuracy and precision of the radionuclide determination by using mathematical calibrations and classically source-based ones in the measurements of specific environmental samples will be presented. As Canberra has developed Laboratory SOurceless Calibration Software (LabSOCS) for this aim, spectrometry setup, consisting of the Canberra broad energy germanium detector (BEGe) with the original lead shielding and Genie 2000, LabSOCS/ISOCS softwares are used for the quantitative determination of low-level activities in few specific matrices such as honey samples and borosilicate filters. Honey samples are found to be an excellent samples to reflect the radionuclide pollution over large areas (such as the Chernobyl derived Cs-137). On the other hand borosilicate filters are used for the measurements of the radionuclides contamination in the air. Accuracy and precision of the setup and mathematical calibrations are also tested with different standard samples of water, vegetation and soil.