The idea of functioning and technical realization of the National Standard Unit of Radionuclide Activity established in Poland by President of the Central Office of Measures is presented in this paper. The Radioisotope Centre POLATOM has been appointed as depositary of that National Standard. The detection circuits as well as the absolute methods of measurements used to realize the standard unit are described. Application of a liquid scintillation technique was emphasized, and the determination accuracy of measurements has been presented. All kinds of the secondary standards, prepared in the RC POLATOM for application in isotope laboratories of nuclear medicine and in industrial and scientific centres, are related to the National Standard by a chain of presented comparisons. Relationship between the National Standard Unit of Radionuclide Activity and the international system of standards has been realized in the RC POLATOM by taking part in international comparisons of the measurement results.