The ion beam optics of a microwave ion gun for a focused ion beam （FIB） system is investigated numerically in this article. Considering the ion beam optics of the gun as a two-stage acceleration system consisting of plasma electrode and Orloff-Swason lens

effects of some primary parameters on the beam divergence are examined. The results show that beam divergence of the system depends mainly on the extraction perveance and voltage ratio

and a minimum beam divergence can be obtained under optimum conditions.