NOVELTY - The assembly has a drive shaft (1) with a rotational axis (21) defining axial direction (22) and circumferential direction (23). A rotor (20) has rotor blades (2.1, 2.2, 2.3) with profiled rotor blade portions (3.1, 3.2, 3.3) and blade mounting portions (4.1, 4.2, 4.3). The blade mounting portions are indirectly connected with the drive shaft in torsionally stiff manner. The blade mounting portions of the rotor blades are directly held in circumferential direction (23) or over separate and spatial separated intermediate elements in alternate manner.    USE - Rotor assembly for an axial turbine i.e. bidirectional flow axial turbine, in a tidal power plant or a wind-power plant i.e. offshore wind power plant. Can also be used for an axial fluid-flow machine such as ventilator.    ADVANTAGE - The blade mounting portions of the rotor blades are directly held in the circumferential direction or over the separate and spatial separated intermediate elements in alternate manner, thus characterizing the assembly by high structure stability of the rotor blade mounting portions and by efficient force and moment transmission on the drive shaft. The assembly is suitable for formation of the bidirectional flow axial turbine. The rotor blade is designed in such a way that simple replacement of the individual rotor blades is ensured. The rotor assembly is manufactured in a simple manner, and resists asymmetric load peaks acting on the rotor blades. The design of the assembly allows reducing notch effect in radial portions of the rotor blades to avoid constrictions in the assembly.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for mounting a rotor assembly.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic simplified perspective view of a rotor assembly in partially assembled state.    Drive shaft (1)    Rotor blades (2.1, 2.2, 2.3)    Profiled rotor blade portions (3.1, 3.2, 3.3)    Blade mounting portions (4.1, 4.2, 4.3)    Rotor (20)    Rotational axis (21)    Axial direction (22)    Circumferential direction (23)