A planetary gear unit (21) of a gearbox of a wind turbine comprising a sun gear (27), planet gears (25, 25′), a ring gear (24), a planet carrier (29) and an outer casing (31); the ring gear (24) being mounted on the outer casing (31); the planet carrier (29) comprising a bogie plate (35) where planet gears (25, 25′) are rotatably mounted on planet shafts (41); the planet carrier (29) comprising a main body (51) and a backplate (53) which can be joined/disjoined to the main body (51) for facilitating the assembly/disassembly of the planetary gear unit (21); the planet carrier (29) being rotatably mounted on said outer casing (31) by means of a first bearing (72) in contact with said main body (51) at the rotor side and by means of a second bearing (73) in contact with said backplate (53) at the generator side.