The efficient red organic light emitting diodes(OLED)that employ Coumarin(C545)as an assistant dopant are obtained.The cell structure is indium tin oxide(ITO)/N,N'-bis-(1-naphenyl)-N,N'-biphenyl-1,1'-biphenyl-4,-4'-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq3):C545:4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB)/Alq3/LiF/Al.The electroluminescent properties of devices with different component ratios of C545 and DCJTB are studied.Compared with devices where the emitting layer is only composed of Alq3 and DCJTB,the emission efficiency and purity of color are improved.We attribute these improvements to the assistant dopant C545 which leads to the more efficient energy transfer from Alq3 to DCJTB.