An electroluminescent device comprises a substrate, a first electrode, a second electrode, and an organic layer disposed between the first electrode and the second electrode, and including at least a light-emitting layer. A metal nano pattern which enables emission of polarized light is provided on one surface of at least one of the first electrode and the second electrode, wherein a grating period of the metal nano pattern satisfies the relation of Formula 1 below. A method of preparing the electroluminescent device comprises providing a substrate, first and second electrodes, and an organic layer including a light-emitting layer, with a metal nano pattern being provided on at least one of the first and second electrodes. Formula 1 is described in more detail in the description of the invention. The electroluminescent device can achieve emission of polarized light, without reforming materials used in forming the organic layer. D