A method for assembly and installation of an offshore wind turbine, where components such as a nacelle, one or more tower sections and/or blades for at least one wind turbine is manufactured, assembled at or transported to a harbor area, where said components are loaded onto a transport and installation vessel and subsequently transported to an erection site. The method involves at least the steps of assembling at least one nacelle for a two-bladed wind turbine having hub with two sets of blade installation interfaces facing in a radial direction, the blade installation interfaces having a 180 degree angular distance therebetween, installing at least one, but preferably two inner blade parts to the hub, and arranging the at least one nacelle with the at least one inner blade part attached to the hub on the deck of a transport and installation vessel.