As a hole-blocking layer,2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is usually used in blue and white light electroluminescent devices.The ability of blocking holes of BCP layer depends on its thickness,and basically holes can tunnel through thin BCP layer.In order to know the role of BCP layer in electroluminescence (EL) of multilayer organic light-emitting diodes (OLEDs),in the present paper,the authors designed a multilayer OLED ITO/NPB/BCP/Alq3:DCJTB/Alq3/Al and investigated the influence of thickness of BCP on the EL spectra of multilayer OLEDs at different applied voltages.The experimental data show that thin BCP layer can block holes partially and tune the energy transfer between different emissive layers,and in this way,it is easy to obtain white emission,but its EL spectra will change with the applied voltages.The EL spectra of multilayer device will remain relatively stable when BCP layer is thick enough,and the holes can hardly tunnel through when the thickness of BCP layer is more than 15 nm.Furthermore,the stability of EL spectra of the multilayer OLED at different applied voltages was discussed.