This research investigates the effect of buffer layer on the performance of organic light emitting diodes (OLED) by adding m-MTDATA as buffer layer between anode and hole transport layer NPB. The devices with ITO/m-MTDATA(d nm)/NPB(40-d nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(40 nm) and ITO/ MoO3 (15 nm)/NPB(25 nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(40 nm) structures are prepared. The effects of m-MTDATA thickness on OLED brightness, current density, current efficiency and other properties are studied. It is found that the turn-on voltage of the device reduces from 13 V to 9 V when the thickness of the buffer layer is 15nm, and the maximum brightness of the device increases from 5900 cd/m2 to 16300 cd/m2, which is about 2.76 times as much as that of the device without buffer layer. The highest current efficiency also increases from 1.8 cd/A to 3.5 cd/A, which is 1.94 times as much as the device without buffer layer. Then the MoO3 buffer layer with the thickness of 15 nm is inserted in the device as a buffer layer between indium tin oxide(ITO) and NPB. Compared to m-MTDATA device with the same thickness, the turn-on voltage of the device with MoO3 buffer layer declines to 8 V, the maximum brightness is 13320 cd/m2, the maximum current density is 6030.74 A/m2, and the maximum current efficiency is 3.06 cd/A.