We have investigated the electro-optical performance of green and red phosphorescent light-emitting diodes (LEDs) with different thicknesses of MoO 3,using BCP as the hole-blocking and exiton limiting layer.The device structure is: ITO/MoO₃(x nm)/NPB(40nm)/CBP:14%GIr1(12.5nm)/CBP:6%R-4b(5nm)/CBP:14%GIr1(12.5nm) /BCP(10nm)/Alq₃(40nm)/LiF(1nm)/Al(100nm),where x i s 10nm,20nm,30nm,40nm and 50nm,separately, GIr1and R-4B are green and red phosphorescent dyes respectively.By adjusting the thickness of MoO₃,we find that the device performance depends on the thickness of MoO_3layer,and we achieve the optimal device.The results show that the device achieves the maximum luminance of 18000cd·m－2 at 12V,the highest current efficienc y at the thickness of 40nm at voltage of 5V, luminance is 100cd·m－2,and the maximum current efficiency is 16.91cd ·A－1.To improve the efficiency of the device,we add the TCTA blocking layer and compare two t ypes of devices,so we can obtain the highest efficiency of 20.01cd·A－1,enhanced by 19%.By analyzing the reason,we conclude that the HOMO levels of NPB,TCTA,CBP are 5.5eV,5.7eV,5.9eV,respectively,thus promoting the hole injection;the trip let energy levels of TCTA, CBP are 2.85eV,2.65eV.The higher triplet energy difference prevents the exc iton leakage from the light emitting layer.