An electroluminescent (EL) device is disclosed, comprising a nanostructured composite electrode, one or more functional layers and an top electrode. The nanostructured composite electrode is consisting essentially of a first layer having a nanostructure and a second layer disposed on the nanostructure. The first and second layers are transparent and conducting, and one of the refractive-indexes of the first and second layers is lower than or equal to 1.65, and the other of the refractive-indexes of the first and second layers is higher than or equal to 1.75. One or more functional layers including a light emitting layer is disposed on the second layer. The top electrode is disposed on the functional layers. Especially, each of feature pits and each of intervals between the feature pits of the nanostructure of the first layer is smaller than or equal to a major wavelength of light emitted from the light emitting layer.