An organic light emitting device, and a method of fabricating the same, has a cathode electrode that can prevent oxygen or moisture from infiltrating. The organic light emitting device of the present invention has a lower electrode, an organic thin film layer and an upper electrode successively formed on the substrate. The upper electrode has at least two-layered thin films having different grain density and grain size. A lower film of the at least two-layered thin films of the upper electrode has a first Al thin film having a lower grain density and larger grain size than an upper of the at least two-layered thin films. The upper film has a second Al thin film having relatively higher grain density and smaller grain size than the first Al thin film.