Heat treatment under different condition on MEH PPV OLED device with single layer structure was performed in this paper.The surface morphology of Al electrode of the device,which is operated at 12 V,is observed by Metallovert.It is found that the bubbles and black spots on the Al electrode surface was reduced,and compared with the unannealed device,the turn on voltage of the device is reduced by 2 V,the maximum EL intensity is increased by ten times,the relative energy efficiency is improved by ten times at the same voltage,the half decay time is increased by 12.7 times.