The optical properties of N,N'-bis (Inaphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) and tris (8-hydroxyquinolinato) aluminum (Alq 3) organic materials used as hole transport and electron transport layers in organic light-emitting devices (OLED) have been investigated. The NPB and Alq 3 layers were prepared using thermal evaporation method. The results show that the energy band gap of Alq 3 is thickness independence while the energy band gap of NPB decreases with the increasing of sample thickness. For the case of photoluminescence the Alq 3 with thickness of 84 nm shows the highest relative intensity peak at 510 nm.