An organic electroluminescent (EL) device has been constructed with a double quantum-well structure consisting of N,N'-bis-(1-naphthl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) as a potential well and emitter, and undoped NPB as a barrier potential. The maximum EL efficiency and brightness reach 5.6 cd/A and 40000 cd/m², respectively. Most importantly, with the increase of the drive voltage, the EL efficiency (cd/A) declines very slowly after reaching its maximum, almost independent of the drive voltage in a wide range from 5 to 13 V. This characteristic may be useful in the improvement of the lifetime of the device.