Illumination system is the key factor influencing both the luminous efficiency and the size of the optical engine of a projector. The traditional illumination system aims at the collection and distribution only for Lambert source, failing to radically overcome the disadvantages of low luminous efficiency or big size. A design in illumination system using Gradient-Index (GRIN) lens arrays for single chip digital light processing (DLP) laser projector is introduced. The complex optical components of traditional illumination system are substituted by single-stage lens. Also, the optical performance is simulated and evaluated by the ray tracing software. The results show that the luminous efficiency of the entire illumination system is 69.5%, the uniformity of illumination is 90.9%, and the volume is only 3.06 cm3, reaching the goal of improving the luminous efficiency and reducing the size of optical engine.