Organic light-emitting devices with a mixture of tris-(8-hydroxyqunoline) aluminum (Alq3) and 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) as the emitting layer have been fabricated. The devices were fabricated in the same run with a standard device without CBP for comparison, with an identical structure of indium tin oxide (ITO)/m-MTDATA (80 nm)/NPB (20 nm)/CBP:Alq3 (40 nm)/BCP (10 nm)/Alq3 (60 nm)/Mg:Ag (200 nm), where m-MTDATA is 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine, which is used to improve hole injection; NPB is N,N′-di(naphth-2-yl)-N,N′-diphenyl-benzidine; and BCP is 2,9-Dimethyl-4,7-diphenyl-1,110-phenanthorline. The ratio of CBP to Alq3 in mixture was varied from 0 to 2. For device with a ratio of 0.5, the current efficiency and power efficiency were significantly improved by 35% and 32%, respectively, compared to the standard device with Alq3 only as emitting layer. By increasing the ratio to 2, the current efficiency and power efficiency were dropped by 20% and 11%, respectively. The electroluminescence spectra showed a slight blueshift with the increase of CBP to Alq3 ratio.