The well structure which was fabricated by alternating deposition of constituent tris-(8-hydroxyquinoline) aluminum (Alq) and 4,4'-N,N'-dicarbazole-biphenyl (CBP) layers,improved the current efficiency. The enhanced efficiency can be attributed to the strong accumulation of electrons and holes in the thin Alq3 layer,which leads to an increase of the exciton formation and exciton recombination probability. The single well device exhibits the highest current efficiency of 4.1 cd/A,which is more than twice than that of the conventional heterostructure device. It is an effective and simple way to improve the efficiency of EL devices by utilizing well structure as the emitting layer with proper well number.