By using an ultrathin 5,6,11,12-tetraphenylnaphthacene (rubrene) layer deposited on the top of host materials， the influence of rubrene layer position on the electroluminescence (EL) spectra of organic light-emitting devices (OLEDs) was studied.When the rubrene layer is located at the interface between N,N’-diphenyl-N,N’-bis(1-naphthyl)-(1,18-biphenyl)-4,4’-diamine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ) layer, EL luminescence of the device is nearly coming from rubrene emission. From the analysis of the EL spectra of the devices with different rubrene layer position, the solely contribution of rubrene and AlQ, respectively, to the luminescence of the devices is determined by spectra unmixing. Based on the analysis, the exciton length in AlQ layer is determined to be about 15-20 nm. The exciton transport and recombination characteristics are also discussed in this work.