Method for installing (or replacing) a unit on the tower of an offshore wind turbine, which unit comprises a rotor including a hub and one or two rotor blades, and a nacelle receiving a gearbox and a generator. The method is characterized by pre-assembling the substantially functional unit ashore or on a platform, placing the functional unit on a watercraft in such a manner that the center of gravitation of the unit lies in the region of the longitudinal axis of the watercraft and the rotor blades extend in the longitudinal direction of the watercraft, transporting the functional unit to the site of the offshore wind turbine, lifting the functional unit from the watercraft using a crane aboard the watercraft, placing the functional unit on the tower of the offshore wind turbine, and fastening the functional unit on the tower of the offshore wind turbine.