A nanostructure semiconductor light emitting device includes a base layer, an insulating layer and a plurality of light emitting nanostructures. The base layer is formed of 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. Each of the light emitting nanostructures is disposed on the exposed regions of the base layer and includes nanocore formed of a first conductivity type semiconductor, and an active layer and a second conductivity-type semiconductor layer sequentially disposed on side surfaces of the nanocore. Upper surfaces of the light emitting nanostructures are non-planar and contain portions free of the second conductivity-type semiconductor layer in order to prevent light emissions during device driving.