The light emitting devices using ruthenium(II)bis(2,2′- bipyridine)(4,7-dimethyl-1,10-phenanthroline) [Ru(bpy)2 DIM]2+ complex as emitter have been fabricated in two structures: Indium-tin-oxide glass (ITO)/poly( N-vinylcarbazole) (PVK):[Ru(bpy)2DIM]2+/Al and ITO/PVK:[Ru(bpy)2DIM]2+/2,9-dimethyl-4,7- diphenyl-1,10-phenanthroline (BCP)/tris-(8-hydroxyquinoline) aluminum (Alq3)/Al. In ITO/PVK:[Ru(bpy)2DIM] 2+/Al, the emitter concentration influences both the turn on voltage and the emission wavelength. At 6 wt% [Ru(bpy)2 DIM]2+ concentration, the device has a turn on voltage of 16 V and the emission peak at 600 nm. At 36 wt% [Ru(bpy)2DIM] 2+ concentration, the device turn on voltage can be reduced to 3 V, with the emission peak at 620 nm. ITO/PVK:[Ru(bpy)2 DIM]2+ (36 wt%)/BCP/Alq3)/Al obtains a quantum efficiency one order of magnitude higher than ITO/PVK:[Ru (bpy)2DIM]2+ (36 wt%)/Al.