We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic lightemitting devices (OLEDs).We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 ℃.Then we performed the tests of scanning electron microscopy,atomic force microscopy,and Kelvin probe to explore the effect of annealing on the pentacene films.The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing.The optimal device with 120 ℃ annealed pentacene film and n-doped electron transport layer (ETL) presents a low tu-on voltage of 2.6 V and a highest luminance of 134800 cd/m2 at 12 V,which are reduced by 26％ and improved by 50％ compared with those of the control device.