The present invention provides an optical device, and the optical device comprises a luminous element and a gradient-index nanoparticle layer and scattering particles composed by particles stack with different refractive indexes and particle sizes. The luminous element has a light emitting surface. The refractive indexes of the nanoparticle layers decrease bottom up. The nanoparticles based gradient-index nanoparticle layer comprises a plurality of dielectric layers with different refractive index, and the dielectric scattering particle layers are stacked upward from the light emitting surface to let the gradient-index nanoparticle layer and scattering particles cover the light emitting surface. The method for manufacturing the abovementioned optical device is also disclosed.