Organic light emitting diodes (OLEDs) containing cathode buffer layers of nanometer thickness were fabricated and their electrical and emitting properties were investigated. The OLEDs have an indium tin oxide (ITO) anode/ copper phthalocyanine (CuPc) / N, N'-dephenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD) / 8- hydroxyquinoline aluminum (Alq3) / buffer layer / Al cathode structure with the buffer layers made from alternating thin films of Alq3 and Al with nanometer thickness. Improvement of driving voltage and the efficiency for the devices were observed by insertion of the buffer layer. It was estimated that some modulations of the Schottky barrier at the Alq3 and the Al cathode interface were induced due to the insertion of the buffer layer and it caused an enhancement of electron injection from the Al cathode. A model of the band structure at the buffer layer was proposed.