Method of mounting the rotor of a wind turbine (11) comprising the following steps: lifting and mounting the rotor hub (15) on the nacelle (21); lifting and mounting the rotor blades (25, 27, 29) on the rotor hub (15) using a lifting device (17) cooperating with a winch (19) on ground, the mounting of each blade being made after rotating the rotor hub (15) for having the connecting part (25′, 27′, 29′) with each blade (25, 27, 29) in front of it when it is lifted, the lifting device (17) being installed in a fixed position in the rotor hub (15) and comprising, in a preferred embodiment, a bracket (31) mounted on the rotor hub front (16) and a hook block (33) mounted on said bracket (31) with a cable (35) connected to said winch (19). The invention also refers to a method of dismounting a rotor blade.