Organic light-emitting devices (OLEDs) with the structure of indium-tin oxide (ITO)/N, N'-diphenyl-N, N'-bis(1-naphthyl-pheny1)-1, 1'-biphenyl-4, 4'-diamine (NPB):4, 4'-N, N'-dicarbazole-biphenyl (CBP)/CBP:bis iridium (acetylacetonate) /2, 9-dimethyl-4, 7-diphenyl-phenanthroline (BCP)/Mg:Ag were fabricated. A doping system consisting of NPB and CBP was employed as the modulated hole transporting layer. The electroluminescent characteristics of the OLEDs were investigated by adjusting the concentration proportions of NPB:CBP doping system. The results showed that the hole transporting capability can be adjusted and the power efficiency was remarkably affected by different doping concentration of NPB:CBP system. Optimized yellow light OLED with a maximum power efficiency of 18.1 lm/W was obtained with an optimum concentration proportion of NPB:CBP of approximately 1 ∶3. The improved OLED performance was attributed to the reduction of hole injection and low transporting capability by doping bipolar host material CBP in hole transporting layer, which significantly enhanced charge carrier balance and electron-hole recombination probability.