The EPR is an Evolutionary high-Power Reactor which is based on the best French and German experience of the past twenty years in plant design construction and operation. In the present detailed engineering phase of the plant under construction in Finland (Okiluoto 3) and in France (Flamanville 3), some actions were conducted in order to improve the knowledge of the hydraulic behavior of the innovative Reactor Pressure Vessel internals (RPV). The RPV internals are mainly derived from former French N4 or German Konvoi with some evolutions to take into account the operating experience. Design and validation of the internals were performed within AREVA’s engineering teams, which develop state of the art methods in the field of thermohydraulic testing. The experimental validation program was closely followed by EDF. Moreover, an EDF R&D project, whose results are not addressed here, was held to consolidate the RPV internals conception. The aim of the paper is to present the hydraulic tests performed on mock-ups to characterize the hydraulic behavior of the innovative EPR Reactor Pressure Vessel internals, and to introduce the role of these tests in the global conception process of the EPR RPV internals (CFD code qualification, design validation, database...). The qualification of the CFD computer codes will be described in a forthcoming paper. Three different mock-ups are presented to illustrate these tests: • JULIETTE for the reactor pressure vessel lower internals, • ROMEO for the reactor pressure vessel upper internals, • MAGALY for the design of the skeleton-type control rod guide assembly.