A coated nano particle and an electronic device using the composite nano particle as an illuminator are provided. The composite nano particle includes a nano particle receiving light and emitting light; and a coating material formed on a surface of the nano particle and having an index of refraction different from that of the nano particle. The coated nano particle is made by coating a surface of the nano particle with a material having an index of refraction, which has an intermediate value between an index of refraction of a matrix and an index of refraction of the nano particle as an illuminator, with a predetermined thickness. The light emitted from the nano particle is efficiently transferred to the outside as the light reflected from the matrix and absorbed by the nano particle is suppressed. Therefore, a luminous efficiency of the illuminator is improved, and an electronic device using the illuminator is provided.