This paper provides the scaling-up methodology that was applied from the BETHSY integral test facility to the real Framatome NPP (Nuclear Power Plant). The ISP-27 (International Standard Problem) transient scenario was used, based on test 9.1b. The objectives were to evaluate the ideal scaling-up of BETHSY facility for real NPP and to compare and analyse the physical phenomena known from experimental background with the phenomena predicted by RELAP5/MOD3.2 simulation of real NPP transient. Further, in order to test phenomenological scaling-up basis two models for RELAP5/MOD3.2 code were constructed differing in scaling criteria for the primary cooling system. Special attention was concentrated on heat structures scaling. Data were analysed through plotting plots and NPA’s (Nuclear Plant Analyzer) graphical presentation.