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 plurality of metal nano patterns are provided on one surface of at least one of the first electrode and the second electrode. 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 plurality of metal nano patterns being provided on at least one of the first and second electrodes. The electroluminescent device can achieve emission of polarized light, regardless of the materials used in forming the organic layer.