A ray tracing model is developed to study the light extraction efficiency (LEE) of the nitride wide band gap semiconductor deep ultraviolet light emitting diodes (DUV-LEDs). The basic device structure is flip-chip LED device with dome shape encapsulation. Various device parameters such as reflecting metal choices, absorption coefficient of the AlGaN/AlN layers, sapphire surface roughness etc have been examined. We have found that the transparency of the AlGaN/AlN epitaxial layers as well as the encapsulation layer play key roles in improving the LEE and spatial distribution. To further verify the absorption by the AlN, highly doped n-type Al0.61Ga0.39N epitaxial layer is grown on AlN template and sapphire substrate by high temperature MOCVD deposition method, then the transmittance is measured after sapphire’s backside being polished to mirror smooth. The extrapolated the absorption coefficient of the AlN is below 103 cm-1, which paves the way to the success of high efficiency DUV LEDs.