Illumination system is one of the most important parts of the micro-lithography object lens. Its performance can greatly affect the lithography machine's etching graphic quality. In this paper, we discuss a DUV micro-lithography illumination system which can achieve high uniformity and a large illuminated area on the mask. According to the large numerical aperture requirement, a refractive illumination system is designed and optimized with software ZEMAX. The system also meets the requirement of large illumination area on the mask, and no aspherical lens is used. Characters of different illumination structures and modes are introduced here. Then by using the software of TracePro, illumination systems with different kinds of aperture are modeling and illuminaces are analyzed. We research effect of illuminace on the mask which bring by different kinds of aperture. Also in this paper, we make a study of relationship between different illumination mode and different kinds of graphics. Finally, we compare the results and give suggestion about how to choose illumination mode. That is meaningful for choosing different aperture in illumination system of microlithography.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.