The refractive index and extinction coefficient are the most important optical characteristics of optical thin-film materials. Optical coating devices with excellent performance are achieved more easily when the selected materials have relatively high refractive index contrast. Here, we used an annealing method to fabricate low-refractive-index material in a multilayer structure originated from the Kirkendall effect. An optical thin film with a low-refractive-index of 1.37 measured at 550 nm was successfully demonstrated and produced by the Kirkendall effect.