We have developed efficient green and blue organic light-emitting diodes (OLEDs) based on co-guest emitter systems. In the devices, two green fluorescent dyes 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1 H,5H,11H-(1)- benzopyropyran-o(6,7-8-i,j) quinolizin-11-one C545T) and N,N-dimethylquin acridone (DMQA) simultaneously doped into tris (8-quinolinolato) aluminum Alq3) and two blue fluorescent dyes perylene and 2,5,8,11-tetra-t-butyl-perylene (TBP) simultaneously doped into 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) were used as the emissive layers, respectively, for green devices and blue devices. It was found that the electroluminescence (EL) efficiency and brightness were significantly enhanced with respect to the single dye-doped devices. The maximum current efficiency and power efficiency, respectively, reached 17.6 cd/A and 13.2 lm/W for green devices, and 6.3 cd/A and 2.5 lm/W for blue devices. The great improvement could be attributed to the effective control of the space charges, thus the depression of the exciton quenching in the emissive region.