The electroluminescent mechanism of organic thin film is introduced. Several kinds of electroluminescent devices (the single layer of Alq; the double layers of PVK as a hole transport layer and Alq as an emitting layer; the double layer of PVK doped with perylene and Alq) are fabricated, and the EL spectra, J V, B V, efficiency of the devices are investigated. By the energy theory, the characteristics change of all devices is demonstrated. When the hole transport layer of PVK is used, the efficiency of double layer device is 10￣100 times higher than the efficiency of single layer device, and the highest quantum efficiency is 0.15%(photons/electrons). The luminous efficiency is 0.08 lm/w, the brightness is changed as the thickness of the hole transport layer and emitting layer are modulated.