Organic light-emitting diodes (OLEDs) are regarded as next-generation light sources. The enhancement of external quantum efficiency of OLEDs has been investigated widely. It is an effective method of improving the external quantum efficiency, which destroys the phenomenon of total internal reflection inside the OLEDs by attaching microlens array to the surface of the glass substrate of the OLEDs. In this thesis, a multi-level aspherical gapless microlens array was designed and manufactured, and it was applied to OLEDs. In contrast with a spherical microlens array, the multi-level aspherical gapless microlens array can achieve a form of high aspect ratio and high fill factor, and they can enhance the external quantum efficiency of OLEDs. At first, aspherical microlens arrays with different parameters, including shapes of curved surface, layouts and feature dimensions, were simulated by optical simulation software, FRED. The aspherical microlens arrays which were attached to an OLED were simulated with a ray tracing method. Then, an optimal geometry and layout were found out. After simulation, a film with multi-level aspherical microlens array was fabricated by a LIGA-like process, including lithography, electroforming, PDMS (Polydimethylsiloxane) micro-molding and UV (Ultraviolet) -cured techniques. The characteristic in this process was to use multi-lithography to fabricate a microlens array with multi-level and high aspect ratio. The shape of multi-level was similar to the design, and the process can achieve the advantage of batch manufacture. Finally, the films with different multi-level aspherical microlens array were attached to an OLED to measure the optical-electric properties. The measured results were compared with simulation and confirmed them.