We have investigated the effect of organic thin film on the driving voltage of OLED (organic light emitting diode) by inserting a 5 nm thick 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) or triphenylphosphineoxide (Ph3PO) between tris-(8-hydroxyquinoline)aluminum (Alq3) electron transport layer and 4,4``-bis(2,2``-diphyenylvinyl)-1,1``-biphenyl (DPVBi) emission layer. The device with 5 nm thick Ph3PO layer exhibited higher maximum current efficiency and lower driving voltage than the device with BCP layer, resulting from better electron injection from Alq3 to DPVBi in the device with Ph3PO layer.