The flow field of a supersonic flow chemical oxygen-iodine laser is simulated by solving three-dimensional Navier-Stokes equations and the effects of the mixing/reacting zone structure on the resulting gain region are studied. It is assumed that the flow is laminar and the water vapor condensation due to the supersonic cooling is ignored. A chemical kinetic model encompassing 21 chemical reactions and 10 chemical species is used to determine the chemical composition of the gas mixture. The present results demonstrate that a pair of contrarotating vortices generated behind the I? jet greatly enhances the mixing and the simultaneous chemical reaction to produce the excited iodine atom with the singlet oxygen. In the present calculation, the small signal gain coefficient is overestimated to some extent as compared with the experimental one. It is thought that the overestimation is caused by the imperfect chemical kinetic model as well as by ignoring the water vapor condensation and the boundary layers on the upper and lower wall in the present calculation.