A Quinacridone(QAD) sub-monolayer was inserted in the Electron transporting layer of Organic Light-emitting Devices(OLEDs),which was used as a probe for excitons′ formation and diffusion.Tthe devices configuration are ITO/NPB(60 nm)/Alq_3(x nm)/QAD(0.05 nm)/Alq_3[(60-x)nm]/ LiF(0.4 nm)/Al(180 nm),of which x=0, 2.5, 5, 7.5 nm is the different position of deposited QAD sub-monolayer. The electroluminescence (EL) spectra and intensity of these four devices show that QAD sub-monolayer located at 5 nm apart from the NPB/Alq_3 heterojunction interface obtained excitons from Alq_3 at most through frster energy transfer.The comparison of the efficiency-operating voltage characteristics of the devices also indicate this result.