A nanostructure semiconductor light emitting device includes a base layer, an insulating layer, and a plurality of light emitting nanostructures. The base layer includes a first conductivity type semiconductor. The insulating layer is disposed on the base layer and has a plurality of openings through which regions of the base layer are exposed. The light emitting nanostructures are respectively disposed on the exposed regions of the base layer and include a plurality of nanocores having a first conductivity type semiconductor and having side surfaces provided as the same crystal planes. The light emitting nanostructures include an active layer and a second conductivity type semiconductor layer sequentially disposed on surfaces of the nanocores. Upper surfaces of the nanocores are provided as portions of upper surfaces of the light emitting nanostructures, and the upper surfaces of the light emitting nanostructures are substantially planar with each other.