Illumination system is a main part in liquid crystal projection display system, and is important to color fidelity and other performance of a projector. As for fly eye illumination system, the light efficiency would be dramatically increased, if the fly eyes lenses are well designed. Two designs of the fly eye lenses are introduced on basis of the analysis on liquid crystal projection displays. The sub-fly eyes in both designs are all rectangular, so they are easy to design and manufacture. The light efficiency could be increased by decentering the sub-fly eyes and changing their shapes. By simulation, the light efficiencies of these two designs are increased by 16.5% and 26.3% respectively. The illuminating light on liquid crystal panel has good uniformity. In the end, two designs are compared with each other.